I voted forrrrrr
Wario: I'd really, REALLY like it if they played up his anti-hero role some more. To be specific, he should appear in a game like Super Mario 64 where Mario collects stars as a recurring character. In some missions, he has stolen a star and you must beat it out of him. In others, he's trying to steal something more valuable and offers to cooperate with you to get it, and gives you a star if you do. Neither the main antagonist, nor a pawn of Bowser's, just a pain in the ass that you have to be friends with.
Cackletta: Out of all the Mario & Luigi games, something about the Beanbean Kingdom and other original stuff in Superstar Saga resonates with the main series, I think. I would've loved to see it and Cackletta stick around.
Tatanga: Basically I'd want him to be the same thing as Orbulon--a funny little alien who tried to be the bad guy but wasn't very good at it. He's so puny. I can see him serving another villain, but with ulterior motives that, to the player, are obviously doomed to fail.