I believe that if Playtonic tried hard enough, they could obtain the rights to Banjo Kazooie and other Rare properties, seeing as MS is not using them at all (and by at all, I mean making new entries in their series). For example, Conker being in Project Spark is cool and all, but the fans of the series want a solid game for him. What Rare managed to accomplish is basically them saying "You want a game with this character, well here is the character. Now you make the game."
I can appreciate that MS and Rare have not forgotten about these characters, but a true blue entry in those series would be nice, not just a re-polish of the older entries (AKA Banjo Kazooie/Tooie Xbox Arcade and Conker! Live and Reloaded.)