Here's my take on it all...
-- Incorporate the concepts of Type 1, Type 2, and Type 3 civilizations. Attaining Type 1 allows you full access to the advantages of your starting star system, and attaining Type 2 puts you out in the galaxy, while attaining Type 3 would be the victory condition.
- Type 1 will be attained through research into "united world government" of some kind. That could be through diplomatic congress like the U.N., or it could be that Hitler won WWII, or it could be that various conditions simply "bred-out" cultural and racial distinctions over time leaving only their thoroughly mixed descendants. Additionally, certain other technologies will have to be researched and implemented such as some form of solar power converter and super-efficient waste recycling.
-- Type 2 will be attained with the development of the first interstellar drive (sub-light only at first, but soon becoming FTL afterward), and will also require the research and construction of "charging stations". These deep-space stations will have vast solar farms tethered to them, for fueling and charging ships before departure. Eventually, it may be possible to research power plants that eliminate the need for these stations. Achieving FTL travel should take just long enough that all systems within a few spaces are close to fully-developed by then.
-- Out of all of that, no more individual worlds...just star systems. Each system has a number of internal "spaces" for various improvements equal to the amount of space available on all worlds and moons in it. The various degrees of terraforming, space colonies, artificial planets, ringworlds and Dyson's spheres would increase the number of spaces available in a system. At the very beginning of the game, a player (or AI) would have only a number of spaces available equal to his homeworld. Upon attaining Type 1 status through research (and probably constructing certain facilities), more spaces would become available, later increased as mentioned before.
-- No more colonization. Building colony ships and trucking them off into the unknown is clunky, unrealistic, and not particularly fun in my opinion. So we're going to make more use of culture borders by declaring that systems falling within your borders are slowly colonized at a certain rate until they become self-sustaining, fully-fledged colonies. This might be represented by a temporary trade-route-like line between that system and the nearest one you control, so that it can be disrupted or eliminated by pirates or enemies. One colony ship is not enough to get a world going in a few centuries, let alone a few years. It would take dozens, hundreds of ships carting pioneers from the developed worlds of your empire off to stake a claim one of many new worlds to achieve that. So that's exactly how we'll do it.
-- Ships constructed as battle-groups. While they'll still be designed individually, they'll be set-up and built as battle-groups (Yes, battle-groups of 1 will still be allowed). Each battle-group would have a logistics cost when putting multiple groups together to form fleets. When building a battle-group, a formation would be selected that provides a particular kind of bonus to some capability of the ships in the group during combat. Likewise, when forming a fleet, another formation will be selected that provides a bonus of its own. Each battle-group will benefit from its own formation bonus as well as that of the fleet's formation bonus. Battle-groups can be repurposed (read: upgraded) to include, remove, or retrofit ships; as well as changing the group's formation. Fleets too can be reorganized to a new formation, have new groups added to or removed from it, or be disbanded entirely of course. While it sounds complicated, building and managing large numbers of combat spacecraft will become much easier this way.
-- Planetary defenses that specifically use battle-groups. Instead of a planet trying to organize its own fleet on the fly out of whatever ships are handy, a battle-group or fleet can simply be ordered to defend a planet or area of space. Anything that attacks the planet is attacked by the group or fleet, and the ships of any race you're at war with that approach within one turn's worth of movement of the group's or fleet's location will be attacked as well. I say "one turn's worth of movement" because there's no real point in having them chase down everything hostile that gets within sensor range. More often than not, they won't catch it, and just as often it'll be a total fluke that they passed in sensor range anyway.
-- Weapons damage types. There's really no point in the distinction between missiles and cannon rounds in the game, and this is true for one major reason... Right now, advances in the rounds being used in artillery of all types are becoming nearly indistinguishable from the warheads of missiles. The only practical differences currently are that one is self-propelled and also their respective launching systems. Even guidance is no longer solely the domain of missile technology. Point-defense technology is likewise developing at such a rate and in such a way as to reasonably target and destroy incoming artillery fire. So it really boils down to directed energy weapons, and ballistics. Consider for a moment that soon railguns will be readily capable of boosting missiles and one realizes quickly that propulsion will be the only distinction left, and it'll be minor at best.
That's all a very long-winded way of saying: the kind of weapon has little to do with the kind of damage it does. So we'll have new damage types; Thermal, Kinetic, and Electromagnetic. Furthermore, they'll be organized in a "Rock, Paper, Scissors" format of damage type to defense type. Thermal beats kinetic, kinetic beats electromagnetic, electromagnetic beats thermal. Ultimately, ballistic weapons should be poor in electromagnetic damage, while directed energy weapons should be poor in kinetic damage. Defenses "beaten" by a damage type provide zero protection against it, while defenses of the identical type nullify damage completely and the remaining defense type reduces damage taken by an amount dependent on the quality and quantity of the defense. Because of this, only two defense types will be allowed on any one ship. This makes thermal weapons and defenses a standard on most ships, with a subsequent specialization in either kinetic or electromagnetic; it also means that it is not possible to design a completely invincible ship and will promote thoughtful and creative battle-group and fleet composition strategies. There can never be an "I win" fleet.
-- Hex maps, 'nuff said.
-- Different economic models. There's trade/finance, tourism, production, and theft. Obviously everything in the game will cost money, except that all money will be based on energy, the one truly universal thing. As such, all races will have power plants that generate income dependent on which economic model they use.
Trade and finance are all about moving money or moving goods, it's about making your empire the "middle-man" in every deal in the galaxy. Races that base their economies on this method primarily make money by buying and selling other people's stuff. This is the most basic kind of economy and one we're all familiar with. Power plants in this model generate a moderate amount of income. Because you can't deal with people who don't like you, diplomacy is extremely important here. After establishing trade between any two systems, The financial institutions of your worlds will provide income based on a portion of your diplomacy. If both systems on the route are yours, this benefit is cut in half (or less). Being a financier and trader is best in a diverse market, after all. This will be the most common economic model among races.
Tourism is a little more unique. The power plants of tourist cultures generate poor income. Tourist cultures "spend" cultural influence to "create" money. Basically they're converters. The more tourist attractions they build, the more cultural influence they convert. This conversion is total, and any "spent" influence cannot be contributed toward expanding borders. This is probably the simplest model of all, but is also the most fragile, as all non-tourist buildings will carry "blemish" penalties for their undesirability in addition to upkeep. This will be relatively uncommon.
Production is exactly that. You build power plants, and they literally make "money" in the form of energy that is then spent by factories and such to "buy" ships and the like. This is about as simple as tourism, but it's the most stable and robust of all systems. This is the equivalent of a "moneyless society" like the Federation in Star Trek. You'll never get rich, but you'll never be poor. This is the rarest economic model of all because it's idiot-proof.
Theft is the domain of conquering races. Power plants of conquering races generate poor income. As their borders expand, they don't colonize star systems, they raid them. As systems come into their borders, various refugees and such flee their worlds and try to make new lives. These worlds provide no benefit to a race using this model until a battle-group or fleet equipped with raiding modules is set to "defend" the system. At that point, a regular income is taken from those worlds as long as the ships remain. This is only barely enough to keep the empire running though. For quick infusions of a lot of money, worlds must be conquered. Conquering a world yields a ridiculous amount of money, once. After that, the world produces only what meager income can be generated by the few poor economic improvements available (usually things like Raiding Coordination Centers), and it can no longer be raided. So conquest-based economics, like in the Roman empire and among the Vikings, can be the most lucrative of all, but has a shelf-life. Good for the first half of the game or so, but then gets progressively harder to keep going as it takes more and more raiding fleets and more and more conquests to keep everything running. This is a relatively rare model for highly aggressive AIs and players.
-- Terraforming and world construction. All of the following technologies have additional series of "upgrades" that reduce upkeep or construction costs.
There should be three grades of terraforming; the first makes an uninhabitable world habitable, the second makes it homeworld-like, and the third makes it ideally suited for your race. Each grade is comprised of 3 technologies making for a total of 9 upgrades. Each improves the amount of usable area in the system by 1.
Space colonies are massive structures housing thousands or even millions of citizens. While any number of them may be built, the upkeep of each multiplies with the number currently built. New technologies allow for bigger and better colonies, improving in 5 grades that each increase the space they provide by 1.
Artificial planets are extreme undertakings attempted only by highly advanced races. Like space colonies, any number of them can be built, and their upkeep is multiplicative, but their base upkeeps are exorbitantly enormous. Additionally, there is only one grade of artificial planet, you either build a planet, or you don't. Consequently, the number of spaces they add is quite high.
Ringworlds Are the first step toward pursuing a status as a Type 3 civilization. Only through complete mastery of stars and their planets can one hope to expand across and utilize an entire galaxy's worth of resources. These system-spanning structures immediately double the available area of a system. Their upkeep is extreme and only one may be built per star.
Dyson's spheres are truly epic devices. Usually constructed at a distance near a star's heliosphere because these structures trap all of a star's output within. Few civilizations can afford more than one of these, and they take a very long time to build. Like ringworlds, only one can be built in a system, but it immediately triples the current number of spaces in the system. Their upkeep can often exceed the output of most star systems. These are the keys to the gateway of becoming a Type 3 civilization.
-- FTL travel. There are three techniques for traveling faster than light. All tend to add up to roughly the same galactic speed, but each has particular strategic advantages and disadvantages.
Shipboard drives are FTL engines built right into the ship. They are the rarest method of propulsion because they're the most complex and energy intensive. They have the advantage of being able to go anywhere, anytime, though they tend to be on the slow side and rather expensive both in volume and credits.
Jumpship drives are very similar to other shipboard drives in that it is built directly into a ship. Unlike those drives though, jumpship drives only go on one ship in a battle-group. They're enormous, and expensive, but jumpships can take many vessels at once along with them, leaving those ships free to pack on more weapons, defenses, and other systems. This tends to be a little faster than individual ship drives, but not greatly so.
Finally there are gate stations. These are massive space facilities that either generate a field that allows a ship to jump, or has an array of gates through which ships can pass to other stations. Either way, ships are restricted to sub-light speeds if they begin their move outside a gated system or end their move out of range of a gate. Starbases may also have gate systems installed to extend the reach of vessels. This is the fastest and cheapest method of FTL travel as each system requires only one gate station, and ships require no special modules to use them; and also because of their extreme strategic disadvantage compared to the other two methods. The loss of a single system or starbase can cripple an empire.
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So thought i,d throw one of my own into the ring: some modifications to the custom races and ship building facilitys. I dont know about anyone else, but if i,m playin a custom made race, made by me, I do NOT want to see ships from, say, the Terran Alliance popping up as my el defacto ships at the start. For example, I made a cyborg race known as the Krylon, and their ships were designed by me to be, well, borg-ish looking. But because you dont get an option to put them into your custom race, (it says choose ship style and race, all the "main races") i chose the Terran Alliance. So once you start up as your custom race, you have Krylon AND Terran Alliance ships.