You don't actually need more characters. There are only three different types of characters small medium and large all characters fit into those and are no more different then simple pallet swaps. Once you've played as one heavy character you've played as them all.
That being said it wouldn't be hard to add more characters to the roster since you don't need create any kind of balance since they'll only fit into the three types. As long as toad is there I'm happy.











