OT's take on the 0 HP idea is quite wise, I think few creatures really qualify for having 0 HP. I'll try my hand at a couple you don't already have:
Crumbling Statue | Crumbling Monument - 2
| 1
2|0 (3|0 upgraded)
Rebuild: At the end of your turn, if your opponent has 3 or more creatures, Crumbling Monument gains +0|+4.
Vine Trellis | Overgrown Arbor - 2
| 1
2|0 (3|0 upgraded)
Symbiosis: At the end of your turn, if you have 4 or more creatures, Vine Trellis gains +1|+1.
I figured making them potentially self sufficient would be a nice twist, and it's much more interesting to have these abilities on a creature with 0 HP.