I don't have a link to it, but Nintendo once said that they could make sequels to games with the WiiWare (didn't they?)
With Mega Man 9 (while not a WiiWare exclusive) they proved they could still make old games.
One of my hopes is for a Super Mario Kart 2. It would have the same graphics engine as the Snes game, more characters (Toadette, Wario, Waluigi maybe), more courses based off of old Mario locales (more than from SMW: How about a Subcon themed course?), definitely more battle courses (we need more like Battle Course 2 and Battle Course 4, though 1 was fun), and more items (bring the feather back, remove the blue shell!)
A track creator would be nice, but I think that would be pushing it..
If they were to make a sequel to one of your favorites, which one would it be and possibly what features would be in it?