I think that for a sequal to work some changes need to be made to the 4x genre. I hope that for GalCiv 4 a game like Master of Orion 1 (MOO1) can serve as inspiration. MOO1 is so simple in design and yet allows for so much strategic decision making and depth. Master of Orion 2 has had a far greater influence on the 4x genre but I think MOO1 is a better game than MoO2.

Another thing that most 4x games have in common is that it is usually clear who's going to win long before the game is actually over. Stellaris for example tried to keep things interesting with end game crisis situations. Another example could be the upcoming game Humankind by Amplitude. Though it's still unclear how it will work in detail, the game tries to adress this problem as well. In Humankind the endgoal is gathering fame and it doesn't matter if you start losing after some early successes, those early successes will still count in your favour.

Good point. I think the game should draw to a close when a lot or most of the content has been seen but it also has to be interesting during that time. In case of GalCiv 3 this could be until you get some late game technologies and had a decent amount of picks in the ideology tree. I find it hard to keep being invested in the game once it's clear that I'm going to win. Once you are at that point the decisions you need to make don't have that much weight to them anymore. In a lot of strategy games (GalCiv included) the earliest decisions you make have by far the biggest impact and are therefore the most important ones that definitely require your full attention. Especially the picks you make when creating your empire have a huge effect on the game. As the game progresses the weight that decisions carry diminishes. Humankind by Amplitude is going to try and spread these decisions over the course of the game by letting you define your empire while you progress through the game. MOO1 adressed this differently be having the player take fewer decisions but have those decisions carry more weight. For example you don't have to build individual buildings on planets in MOO1. I know these buildings could be fun in the early game by the late game this would slow you down with a ton of decisions that don't really matter at that point.

Another reason the importance of the early game is magnified is the nature of 4x games and their economy. The economy in a lot of those games is mostly a zero sum game like how it was in real life before the industrial revolution. This means that taking stuff of a opponent, especially in the early game is a huge boost. Most economic development is locked in specific ways because of the rules of the game, i.e. fixed population caps, building slots, fixed number of resources/products, etc. If you would start with only half the amount of habitable worlds as your neighbour you are in trouble. Most games don't allow your economy to freely develop because of the imposed constraints. Some games tried to solve this with "tall" and "wide" playstyles but those still come down to artificial rules that only allow either of those styles and nothing in between. It might be hard to change this in  games but some advantages wide currently has in most games are for example: populations don't move, empires don't break up internally, distance is no factor in production/research/communication/politics, no need to divide your forces across your empire.

Too summarize, as you progess though the game the importance of your strategic decisions diminishes. If your strategic decisions no longer have any weight, why should you continue to play a strategy game? I think that in 4x games the emphasis is too much on the last x, exterminate. All the other x's only serve in support of the last one. Perhaps this could only be solved by redrawing the whole concept of victory conditions. I think if GalCiv4 wants to do well it needs to build on  the strengths of it predecessors while also bringing something completely new to the table.

I'm not arguing to remove planetary buildings unless there is a far more interesting and engaging system to replace it. I'm only arguing that by the late game it adds a whole layer of busywork that has very little to do with actual strategic decisions. Perhaps the way the game works needs to be re-drawn to make the biggest maps a bit less tedious to play. Master of Orion 1 doesn't have all this busy work and let's the player focus on a higher level of decision making instead. 

I'm also not arguing for a tall/wide solution, I'm arguing against it. Most solutions that have playstyles that cater to this allow only the extremes and nothing in between because of rules imposed by the game. I'm in favour of a way to have a more gradual scale for both those extremes. If populations could grow without a lot of contraints over the course of a game and would have some ability to move on their own would make a big difference. Population should be your most valuable resource. Look at a game like Victoria II, in that game single provinces can have a higher economic output than entire low populated countries. Economies in how they work now are not zero sum but positive sum. It's far cheaper and easier to start a business that sells a product that is in demand than to take resources by force.