Less of a niche title? Absolutely. I don't think it's going to be some massive system seller, but it's 3D platformers are absolutely less niche than a fighter. Plus, to me, it makes the most sense. They're going to have oodles of shooters, so Perfect Dark is out. KI, I feel like is coming, but I think they'll want to saturate the market a bit with Xbones before it hits. That leaves Banjo, or Conker.. And I just really don't see them launching with Conker.
I think Nintendo platformers are more popular today, but I think in general the fighting game genre has more fans and people interested than the (non-Nintendo) platformer genre, not to mention it's a big in for the pro-gaming scene that Microsoft seems to be interested in catering to.