Wow, there were a bunch of ideas floating around in another thread (which was off-topic from the original thread), but this was just what I was looking for! Sorry for long thread!
Here are some ideas as far as Diplomacy goes:
1. Exchanging War Plans: If you are allied with another AI (only if you are allied), you should get the option to exchange battle plans. For instance, the AI would tell you what planet or planets it is currently attacking/or planning to attack. The AI would perhaps make a suggestion for you to attack another nearby planet. You could present the AI with another target, if you wish, and the AI would tell you if it is willing to do so or not. Also, if an ally is having trouble taking a planet, it may ask for assistance, and you can do the same. Having allies would become advantegeous to a much higher degree when you can 'somewhat' coordinate attacks. This would not be intended to be some super high-degree AI requiring thing, just a simple diplomatic option.
2. Territory Violations: This was discussed in another thread, not sure if it's a good idea or not (most likely not, I could care less to see it), but say you are on 'bad terms' or a war just ended with an AI, then they can request you leave their territory (only for the ships they have 'spotted' though). Since space is so big, this isn't really a great option though. To have this work, Eyes of the Universe would need to be scaled down (as it probably should anyway). Plus your Influence borders really aren't territory so-to-speak anyways.
3. Truce: If at war with another Civ, and both sides decide to make peace... a short-term truce should be enforced by the UP (# of weeks is up for debate). Or better yet, perhaps only if this is voted on during a UP vote. I know something similar exists where you have to wait 1 week after declaring war, and one where your ships are moved to neutral zones.
4. Alliance Bonus: Same as they are now, but for having an alliance, both parties should gain a small bonus (such as a small economic bonus, or increased trade monies, since it would be seen as an Ally is someone the other can trust, and therefore trade and economics are slightly increased). This could be a larger incentive for AI's to make allies.
Idea for Winning the Game:
1. Economic victory: This one is just asking and begging to be included. After all, an absolute economic powerhouse is something to be feared, with so much money, there is nothing out of reach. (except that tech way way way down the tree). E.G. to win an economic victory, you would have to obtain a certain amount of BC in your treasury, and then retain that amount of BC for a certain number of turns (would have to be a high BC amount and a pretty long period of time). All other AI's would be notifyed and would all attack to stop you from winning (similar to ascension).
Mega and not-so Mega events:
1. Asteroid strikes planet: (damage to a tile), not the asteroid destroys planet, as they have now.
2. Blight event: a strange Blight breakout occurs on a starbase, you must bring a Constructor there to build a special quarantine to save the base.
3. Solar Flare/Sunspots event: Solar Flare/Sunspots from a star (if you have a planet there) causes either some disruptive problems, or a possible benefit (not sure how this could be beneficial, but hey) if you are high up in the tech tree.
4. Industrial Boom: Similar to the economic and tourism boom events.
5. Collaspe events: Similar to the boom events, but would be the opposite, an economic collaspe would make economics go down (these should be very short term only). They already have this for influence, the xenophobic event. Could go for trade, industry as well.
6. New Resource discovered: A random new resource is discovered in the vastness of space, could be any of them (morale, economics, military, etc), and anywhere on the map.
7. RARE Supernova/Nova event: During a game, a random star could go Supernova or Nova. This would be seen coming though, and a message would display saying the star will go in 15 turns or w/e. If you have a planet/planets there, you would need to evacuate all citizens of those planets, or get a diplomatic hit for letting them die. Evil civs would get no diplomatic hit. Other inhabited planets nearby may take a small production/economic hit from the supernova radiation. The Supernova/Nova will then become a neutron star or black hole, and may produce a nebula. Note: Supernova's occur in Milky way once every 50 years about, so 0-1 times per game is enough.
8. New major race: Possible new race enters the galaxy at one of the edges or through one of the wormholes that are laying about. Similar to minor races appearing in ToA. These would enter with higher tech levels than most tech advanced player, and game would allow them to colonize any world INITIALLY ONLY (in case all worlds are taken).
To be honest, I love random events! They add a whole extra dimension to the game, there are so many possibilities, I hope they do add bunches of em in GC3.
1. Too much industry can cause planet damage (ecological dam is the word I'm looking for). This is more of a reality than anything, just look at Earth. Although this may eliminate strategies such as the all-factory build and such, so just an idea. If it stays high enough for a long enough time, a planet tile could be made unusable.
2. Too much research on a planet can cause citizens to go brain dead. J/K! Because this would eliminate the all-research build strategy.
2. Better Planet Defenses: I know Stardock has said they will not add certain things into GC3 EVER! But IMO, planets are too easy to take, it doesn't seem as if you have to actually 'WORK' for them. Making the AI build the defense improvements more often may help this. As well as making the citizens of worlds a 'bit' tougher so they don't get killed so quick. Having planets use stronger defending vessels as well.
Spice up Space!!!
1. Nebula: Nebula can occupy some parts of the map. These have a HUGE amount of uses in terms of gameplay. Nebula is an interstellar cloud of dust, hydrogen gas and plasma... so sensors would not work in them. Good for hiding places, to escape an enemy, surprise attack. Ships cannot be seen in nebula either. Ships could still move, say half speed or more (dense amount of particles interferes with 'warp' engines or w/e). Occasionally (ANOTHER EVENT!), a nebula may form a new star, and possibly a new planet (since that is what they do, would have to be a rare event though, maybe 0-1 times per game). The Supernova event could also leave a nebula.
2. Black Hole: in center of the galaxy: Perhaps on the largest maps only (immense). Just for kicks really, wouldn't have much of a purpose, unless you want to fly a ship into it to see what happens. Of course, initially you won't know it's there (accidently fly a 400-25 dreadnaught into it, that would suck).
3. Voids: these already exist in games with tight clusters...
4. Dark Matter Voids: Be wary before entering this area...
5. Terraforming: Terraforming unusable planets to usable planets. I could say perhaps on a very few planets... but once terraforming started, it would take a game year to complete or something before it's usable. Your terraformer spacecraft would continue it's mission, draining manufacturing hammers and commerce every turn from whatever planet you wish it to come from (and would be quite expensive).
6. Areas of High Radiation: These areas will steadily eat hp's from your ships if you stay too long. Areas could include: Certain nebula (not all) or parts of a nebula, in an radius around the star that supernovas for a time period, certain stars, etc.
There was a discussion at https://forums.galciv2.com/334511/page/3 about cloaks, which could probably work without them being overpowering. Also, evil civ's or maybe as a special attribute, could have tractor beams, to attempt to takeover a ship (with limitations though, perhaps only up to small hulled ships that are not in fleets).
Well, that's it for now, message is much longer than I meant to make it... I'll add more as it comes to mind.