I've played through all of the Wario Land games aside from the Virtual Boy installment (yes, even Master of Disguise). It's a wonderful series and I like the way that it differentiates from Mario with the increased focus on puzzle solving.
Actually now that I think about it, with the way 3-D Mario has shifted during the past two cycles toward a return to the linear obstacle course-style games (starting with Galaxy, and pushing that even more directly with 3-D Land/World), Wario might be a weirdly perfect fit to slip into the role of the MS64 or Sunshine style of game, featuring larger worlds with a bigger emphasis on exploration, puzzles, and enemy interaction rather than straight up platforming. That would be assuming they wanted to even make a 3-D Wario game, I'd be more than content if they just kept with the style of the 2-D games, as I've loved all of them (except, again, for Master of Disguise).
Whether Wario or not, Nintendo NEEDS to get another game made in Shake It!'s art style though, the hand-animated work in that game was so gorgeous and made even some of the game's duller moments a treat to me. The lovely soundtrack helped out a lot, too.