... Mario and Luigi RPG with a job system.
That's something they haven't yet done. With a party of four/five. I'd love those two with their princesses (and Rosalina) to work together lilke that.
Oh, and Wario and Waluigi, with their own loves (if any). Should be silly.
The problem here is that the combat system would have to redone to accommodate the change unless the characters all have separate gameplay (like Bowser and Mario/Luigi in the third game).
Also Wario and Waluigi are the antithesis of Mario and Luigi, so love would be out of the question though personal greed isn't.