I think the animal friends are interesting because they not only alter your ability, but they can also be mounted by any character.
This would allow the other characters in multiplayer to have some more moveset variety like Kirby, even without the copy ability.
Also, because they modify the way you interact with the environment, changing the gameplay a bit (not just the moveset, also the physics and the way of platforming).
And last, because they are characters, not just gameplay gimmicks, and I miss them as characters too.
I think they should be like Kirby in Smash: they should have a fixed moveset, that will remain unchanged no matter what ability you have, but only one of their moves is altered depending on the character you are using or the ability you have with Kirby. Like Kirby in smash, he doesn't change the full moveset but just a single move, B neutral.
Also, aside from the main 3 animals, I would like to have more animals. Not necessarily the other 3 already seen in DL3 (Pitch was almost useless to me XD) but maybe other animals, new ones, or other characters converted into mounts (the return of the Wheelie?).
For example, Pon & Con, a Nruff (Wouldn't you like to see Bandana Dee riding a Nruff?), or even Landia (or the 4 legged tank from the early concept of KDL2, it's not an animal but it would still be cool).
The 3 main animals should remain forever, but I wouldn't mind having different extra animals each new game.
