Banjo got in and they’re owned by Rare, who is owned by Microsoft, so your point is moot. Yes, Banjo DID appear on Nintendo consoles, but then, Snake got in back in Brawl, not even in Ultimate, and he returned in Ultimate, and afaik Metal Gear has NEVER been on Nintendo consoles.
Master Chief, Steve, and Banjo would represent Microsoft’s most iconic (in my opinion, only real exclusive) franchises, and of those two that aren’t Banjo, I think Master Chief is more likely because he would be a zoner/gunner type which is none of the DLC thus far’s archetypes, meanwhile Steve would be a sword similar to Link.