I'm firmly of the opinion you should only add more modes if it's something you want to do, since it means EXPONENTIALLY more work, but if you decide that it is, there's three I can think of that would likely appeal to the widest audience. As mentioned in a few places, the addition of Udders and their expansion would be thematic; I know a small but very dedicated group who would very much appreciate ass/hip/thigh expansion; and lastly and least well baked (but perhaps simplest to implement), some kind of all-around expansion, which could be as simple as making multiple toggleable at the same time (and double checking nothing clips inside anything else)
As always, thank you very much for making and working on and polishing and updating this game.
PS: I know you said max two, but I figure it's best to give you the options and let you make the decision :3