Here's one possible idea for the black common aura slot, that uses an opponent's Discovery against them.

Nice idea, but I think I would rather play Quag Sickness. This might be pretty hard to kill anything with, which makes it a feel bad card most of the time.

I don't like the idea of discouraging discovery so explicitly. Counters are one thing, this is another.

Was worth a shot. We'll need some other idea for this slot then.

How about just a sacrifice Aura?

B Enchantment-Aura The next time enchanted creature untaps its controller/owner sacrifices it.

Not surefire way to kill but shuts down utility creatures and hampers unbound a bit.

That's not a bad idea in the way it screws with unbound. Though in a way it's just a baiting/disincentive way to do what (((Asphyxiate))) does. But the function is different, in effect it's equivalent to "enchanted creature can't attack or tap" (unless its controller is willing to sac it next turn), so it does feel quite different.