I think it would be entertaining if Jellies could combine to become larger (like Lichen) or perhaps to be different it could start as a big slow cube that when sufficiently damaged would break into 8 normal sized ones. Then you could go for a 3^3 that would split into 27.
My computer would probably grind to a halt, but a 4^3 Stone Jelly would make for a diabolical boss battle or a danger room.
Browniebear
I think the Green Jellies split apart.
They go from one huge one, to some smaller ones, then smaller ones again.
I don't know if they're in game right now, though.