I have a couple ideas ideas for new babies!
(Note that I haven't 100% the game yet, so if what I suggest is already in let me know)
More heart producing babies would be incredible. An equivalent of the primal sunflower in PVZ2 would be much appreciated, as they make early game more difficult but greatly improve mid and late game. Another idea I have is an expensive shooter baby (maybe 3-4 hearts) who makes light beams upon hit, and every time it kills an enemy it produces a heart or two, depending on how balanced it turns out being.
Aside from that I don't think too many more babies are needed, the game is already surprisingly well balanced
BUT, perhaps a character selection system similar to PVZ Heroes would be implemented? It'd help give the player more opportunities to choose how they want to play, and reduce the chances of RNG screwing u over!