Of all the classic characters who deserves a comeback, I'd say it's Tatanga. Cool name, great redesign potential. There are no other alien regulars. I don't think he has much potential in the main series, which is mostly what I care about, but he'd be a better Kart character than Birdo... So would E. Gadd... Especially if they went in the Mario Kart DS direction* again, with everyone having a selection of three customized karts. Gadd would have a field day with that, and Tatanga would obviously have karts that resembled spacecrafts, maybe even a hovering kart.
Super Mario Land was a dreadful game if you ask me though.
*How on Earth did they go from Mario Kart being as in-depth and innovating as to give every kart an item rating to being so flat and boring that your character is almost a featureless pawn? After unlocking the option to seat any character in any kart in DS, it becomes that way I guess, but the lack of personalized karts in MKW is a huge let-down. Sorry to get off-topic and complain about something unrelated to the original subject; I am the only one around here who does that and I am not aware of how annoying it is, especially since I complain about the same things repeatedly.