Even with the news flurry today, I still think it'll be Kirby. It's really one of the few things that'd fit all the clues (release soon, big-ish franchise, nice surprise but nothing core gamers would get that excited about) and the GameFAQS 3/20 release date is just too weird a coincidence.
This is sort of how I see Nintendo's "big guns" JP Wii schedule going now...
Hoshi no Kirby (3/20)
XenoBlade (Golden Week)
Metroid: Other M (summer)
Super Mario Galaxy 2 (early fall)
The Last Story (late fall)
Zelda no Densetsu (December/holiday)
...with Wii Relax (or whatever it gets called) also squeezed in there somewhere.