You know...this is an interesting question.
For me, as someone who hasn't bought a Wii U, this question is more like "What new game would make you buy a Wii U?"
And as much as I hate to sound like some fanboy, my guess would be Zelda. Link's first HD adventure would pretty much be the only game announcement that I would have to play. I've played every single Zelda (except for CD-I but I would play those if I could, for shits and giggles), and I'm not going to stop anytime soon. Unless there is a clear drop off in quality and fun, then maybe I'll forsake my favorite series, but until then...I eagerly await the Wii U version.
If there was any other game, it would probably be something I can't even begin to speculate about. Metriod would definitely peak my interest, but in order to pick it up day one, it would have to be something phenomenal. Like MP3 but more ambitious. That's a pretty high bar I just set.
Pokemon would also peak my interest. I haven't played a single one since Red, but I always liked the formula. If they made it like Xenoblade, where the pokemon are just enjoying their lives in a massive landscape, I would pick that up day one.
What else? I'm really not sure. It would have to be pretty amazing, whatever it is.
EDIT: But let's be clear, I will one day own a Wii U. That I am sure. But when? It depends.