New DLC would seem to be a great way to sell a 3D upgrade. But as for what would be a good theme, I really don't know... A "classic" DLC with older tracks and such wouldn't help bring the nostalgic/regular fans into the game (we're probably already playing HD & Fury), and has no value for newcomers. Racing is covered in HD and battle modes in Fury. What else is there to WO other than that? New modes could stretch the core of the game too thin.
I wouldn't hold my breath for a new themed DLC, even less for a whole new game in the series in the near future. But the nature of downloadable content is still in the air, so who knows...
If SL tacks a couple of new/old tracks on the 3D update, I'll be happy to fork over it-- even if I have no use for 3D.