Not sure if they're already in the game or not, but it would be cool to have blue flies in the game. Maybe when a Mulligan's mask is broken, if you manage to kill it before it escapes it can produce three blue flies. They'd home-in on an enemy and hit it once, killing the fly in the process just like in Isaac.
I also agree that the number of slots for babies would be nice. I've got so many babies, but C.H.A.D. and cube of meat are almost completely necessary.