He was supposed to sleep for 200 years in the TV anime...
Still, I could think of about a thousand characters for a Mario spin-off, and before I go on, let me reassure you that Wart is not lame. If they brought him back, he could be the tactical anthesis of Bowser, with water/bubble breath as an attack. Stanley the Bugman you're right about, thought I will admit that he deserved an SSB cameo.
Hell, there are even some enemy drones that deserve to be in more than the babies. What ever happened to Chargin' Chuck? Where's the Snifts? How about a friggin' Raphael Raven at least?