Probably a controversial suggestion, but honestly one of the best things about the original pvz is the fact that you can always choose your towers depending on the situation and once you unlocked them you have them for good, with no stress of losing them all and having to start over from the first four, so my suggestion is some kind of endless mode, in which the player has all babies they discovered so far unlocked from the start, the catch is that they can't discover any new items, active items, or new babies while in this mode.
Again this might be controversial since this game is supposed to have a roguelike gameplay, but I think that would be nice for people who enjoyed the plants vs zombies feeling of having your arsenal of towers unlocked at your disposal with no stress of losing them all.