I posted about this recently, but I thought this deserves special attention so I've made a main post about it.
I get that most of the work done has been about core mechanics, and all of that work is amazing. However, I think that considering the tech-tree content might give foresight into more core mechanics work later on. (if you have a great idea for a tech-tree, but it requires you to make a new menu, or AI programming, for example)
Basically what makes these games fun for me are the customization options we have for our society. When you're able to personalize something in the game, you own that and it makes you get more involved and immersed in the game. Currently the tech-tree's are very linear and do not allow much personalization. I can choose 2-3 technology branches in either Civics, War and Magic, but they are interwoven so choosing which one is really inconcequencial in the long run.
It would be nice to have a sub tech-tree for each aspect of the game. One for each kind of spell, maybe one for each " combat style" of the society, one for unrest, etc. Make it so that it would be very unlikely you'd fill the entire main tree.
Basically make it more like Galactic Civilizations 2, but hopefully more diverse.
I hope this is helpful!