I think that if this games existed, it would be the smallest roster of characters ever in a fighting game.
True, Microsoft needs to up it's game when using the characters it's acquired but almost never used.
Here's what they gave Banjo-Kazooie for their 20th anniversary; Basically nothing.
But on the topic of roster, it would go like this.
-Banjo and Kazooie
From Killer Instinct:
From Viva Pinata:
From Perfect Dark:
From Conker's Bad Fur Day:
From other Rare games:
-Juno (Jet Force Gemini)
-Sabreman (Saberman games)
-J-Bomb (Blast Corps)
From Gears of War:
From other Microsoft owned games:
-Vince (Voodoo Vince)
-Ori (Ori and The Blind Forest)