People
Why not just expand the main building? You could make it cost just and increased amount if resources, and upgrading it past lvl 20 would only increase the amount of people you make and can have. Build time isn't decreased past lvl 20.
If someone has suggested this, I have not seen it yet.
Please tell me what you think!
~jeebsterz
P.S. This will NOT cost coins as it would make the game very unfair for some