I highly, highly doubt Steve will be a character. To represent him faithfully would mean he'd have basically no animations or expressions. It'd be boring. To make him function as a full character, there'd need to be a lot of creative license, which with third parties is always a problem. Granted, I believe Microsoft is chill enough to allow Nintendo to do stuff like that (as opposed to companies like Square Enix), so it could happen.
I feel like if Banjo is added, Nintendo and Microsoft might seize the opportunity to add more than just Banjo and get some more Microsoft stuff. Like how Capcom got both Mega Man and Ryu, or Konami with Snake and Castlevania, and allegedly SE with Cloud and whoever else is coming. If there will be any exception to the "1 character, 1 stage" rule, I can see it being with a historic collaboration between console rivals.
It's the best hope Master Chief has, at least.