Probably something that offers people a new experience (possibly with a peripherial)
UDraw could be that thing, if it's marketed right, and is fun. Not sold on it yet.
I'd say NHL Slapshot - could well be in Canada - despite whatever they score it, critics says it great fun - but hockey isn't big enough elsewhere to be a breakout hit.
Create has an outside chance, but I think the Wii version is gimped (online) which would stop it from reaching it's potential.
Really I think Wii Party will be the BIG breakout game this year with that crowd.
Whatever else ends up being big, will be interesting to see.