Here is my prediction for the next series.
It will happen in 2011, probably over the summer months. There will be five games this time around instead of seven, which I believe will be:
Mario Golf Toadstool Tour
Because we very likely won't be seeing a sequel. Will have support for Wii Motion Plus (if it's successful... which it will be).
Luigi's Mansion
Another title that's unlikely to receive a sequel. Perfect fit for the IR pointer.
Super Mario Sunshine
I don't think it performed very well it Japan. Plus it's Mario, guaranteed million-seller. Another good fit for pointer controls.
1080 Avalanche
Again, sequel seems dubious. Would support the Balance Board (optionally of course).
Star Fox Assault
Seems like yet another good fit for pointer controls. Might either be released after a new Star Fox or to build hype for a new one.