I'd love to be able to build a building and have it trigger some sort of event. The game already has some hardcoded events (creating your first Medium Scale ship, your first trade route, your first invasion, etc).
How neat would it be to build the Temple of Righteousness and have it trigger a bump in Planetary Quality? Or a Temple of Debauchery and your races Population % increases...
You get the idea, I'm sure.
Also, I'd like to see the 'extra' tiles (the ones you have to soil enrich/habitat improve, etc) have a chance of being a bonus tile (bonus to morale, research, etc).