Air: Dive could easily be OP, especially with something like blessing. Maybe its ability could be shockwave, which kills itself. But it wouldn't be able to use the ability without a buff. Seems UP though. Maybe just casts shockwave upon death?
Darkness: It can survive without any buff other than eclipse, so this has to be careful. Growth is probably OP, as it's all mono element and darkness can already boost attack.
Death: Something like "for every poison counter applied to this creature, its hp goes up by one. Cannot take damage from poison" would be cool. Maybe its attack goes up by one too.
Earth: Hmm. Protect artifact is interesting. Prevents a permanent from being destroyed for as long as this creature is on the board. That might be difficult to program though. I don't think it would be OP, as you'd need a buff and a quint to guarantee that the permanent is permanent, while you could easily just use an actual protect artifact. Ugh UP? No other ideas. Have to be careful though, this can be mono buffed.
Gravity: Its skill should just be momentum, like charger.
Life: No idea either. Possibly adrenaline, but nothing really makes sense with it.
Light: I was thinking the perfect 0 hp creature would be photon, but that's taken. I don't think there can be a good idea for this unless it's similar to the entropy one...
Time: I think something like "when this creature dies, it returns to your hand" would be awesome. OP though. Though maybe not if its cost is around 3
and its attack is 2ish. Hmm. I like that.
Water: Every turn this creature is alive, remove a poison counter from or add a purify counter to yourself. This wouldn't be OP with fractal because of the buff cards necessary I think...