I would like the ability to build over a "one per planet" building.
I can see two examples of this happening:
1. Conquering a planet. I've seen some crappy buildings when I take over another civilization's planet. I can't think of any specific examples, but it would be nice to have the option to remove it.
2. Accidentally building the wrong building (or being more of an idiot). This may just be me, but I've accidentally hit Manufacturing Capital instead of Manufacturing Center. Since this is a project, it may not work the same way as other buildings. I played Krynn for the first time last night, and I got the bonuses backwards (I thought morale + economics instead of influence + economics) and built a Consulate on an approval tile. It would have been nice to build an Entertainment Center over it, then build a Consulate on a different tile.