Okay, I just made a considerlist. I don't want to forget the ideas I get, so I'll probably edit the considerlist in the first post with changes I might want to make when a game is finished. Here the current one is:
Considerlist
Artillery/aircraft squares (from turn X, artillery is enabled, so they can destroy enemy squares developed within a radius of Y, from turn Z, they turn into aircraft squares, so they can invade those squares from a distance).
City squares which can be developed to different levels.
Discovering parts of maps mechanic.
Economic victory condition (own X resources to win).
Diplomatic victory condition.
Pillaging (defeating a nation gives you X% of it's stored resources).
More valuable extractors (increasing the bonus they give).
Dividing square zones into multiple squares, so they can be developed to multiple levels, and only one level pr. turn developed/destroyed.