The horribleness that is Kinect Sports sold well, and avatars are pretty successful, I don't see Microsoft letting go of Rare, unless Nintendo offers big, which I also don't see.
What's a little more possible is that the IPs get sold off if Microsoft thinks it'd be a better investment to sell them to Nintendo than what they could make on their systems. Again, Nintendo would need to offer something decent. Sure, Microsoft is basically letting Conker, Perfect Dark, Killer Instinct (though they tried to renew that one), and especially poor B&K die slowly, but they probably still wouldn't be eager to let potentially valuable IPs go without decent compensation. And honestly I don't see Nintendo in a rush to scoop them up. They have plenty of other profitable IPs.
But if Banjo came back... it would be glorious... I'd probably break something...
And unless Sega actually starts legitimately going bankrupt and looking for a buyer, I don't see Nintendo picking them up.