I agree, M&L&P&B(&Y&W) is enough. Which isn't to say there aren't characters I'd like, just that I know the difference between what I'd think is cool and what's actually a good idea, and if I didn't, I'd be trying to convince everyone of why Politoed and Quagsire would both be great additions, at the same time.
If I had to pick one more Mario character I'd go with E. Gadd, with a moveset based entirely on what Luigi can do in the two Mansion games. Either that or... I'll admit, I don't actually dislike the idea of Paper Mario in his own spot, if his moves were heavily based on the first two games... Hammer attacks, items, special jumps like the Tornado Jump, partners assisting in specials. And then his Final Smash could be Bump Attack, Supernova, or even a partner character's ultimate attack, like Spiny Storm.