There's a lot of spin-off potential in Mario games, at least when it comes to giving other characters a chance to shine, but it's almost never used. Like Luigi has 5 games to his name, one of them is an "Edu-tainment" title, and another is DLC for NSMBU. Peach has one, count 'em, ONE game of her own. So expanding that further with guys like Waluigi or Rosalina is unfortunately into pipe dream territory.
Wario and Yoshi got the most success out of it though. With plenty of their own games, platformers even, with different mechanics and physics that simply wouldn't gel too well with Mario's style of 2D platforming. I mean, it'd be cool if Knuckles or Tails got to headline a different game where the platforming is based more on their strengths rather than just speed, for example.
Some other platformer series handle this in a different way. Like Mega Man, instead of giving certain characters their own game, most of them feature a different cast altogether, with a new Mega Man as the lead. Except the Zero series, where Mega Man X acts like Dr.Light/Zero in X1-X3, strictly as a support character. Outside of that, the closest we got was Mega Man and Bass and that one T. Bonne game.
The real reason a spin-off should exist is to make use of mechanics that the main hero might be able without alienating the fans of the normal games. Mega Man is too weird with wall-jumping and upgrading health? Go make Mega Man X instead. Mario ranking you by coin count and not clearing the level or only dying by time out too weird? Go make Wario Land instead.