Super Smash Brothers Ultimate - PLEASE, PHIL AND SAKURAI, YOU MAGNIFICENT BASTARDS, YOU.

I wouldn't actually be surprised if it does happen. Nintendo and Xbox have gotten pretty buddy-buddy during Gen8, to the point that there are now a couple games where Nintendo version owners can cross-play online with Xbox version owners – Rocket League and soon Minecraft. Not to mention the fact that Microsoft owns Minecraft now and yet it's still hitting and getting updated on Nintendo systems.

Come to think of it, there were plans to add the duo during the early Melee days but time constraints and the sudden Rare buyout prevented it.