Just another idea that someone might make good use of it.
Having a flying unit over a mineral block (like, 2 overlords) so you will have to first kill the aerial unit before you can mine that path and get out of the way. That's a way to delay even further expos or certain routes, until they have troops there.
Guess they'd work better on island expos since terran get marines just in the beginning of the game, while P and Z have to wait for goon/sair hydra/muta.