Give the player a probe, and lights, when they construct a specific protoss building, remove and replace it with the zerg building of choice. The limitation is that you cannot stack on it again after you see it.

I don't understand what you're getting at here... So I have a probe, and I build a Nexus. The Nexus is then replaced with a Zerg Hatchery... then what?

then you can keep building more zerg buildings there until you turn the lights back on!

You can stack as many nexus's as you want and they all get replaced with hatcheries.

Wouldnt the creep stop you from building a second nexus?

Not if your lights are out, if you cant see the creep, sc pretends it isnt there.

and make ground out of tileset creep.
That prevents the creep of spreading and you can build new toss or terran buildings everywhere where you do not have a zerg building.
But the lights have to be off to build on tileset creep.