I'm sorry, but that is one of the dumbest ideas out there.
Nintendo as a brand is a far more valuable than Xbox as a brand, right off the bat.
And what would Nintendo gain by owning the Xbox brand? A few more loyal fans?
What MS exclusive franchises are there, that really would matter that much in the long run? Halo, Gears of War, Forza, Kinect nonsense. Getting Rare back, on paper, would be something, but consider it's a hollow shell of what Rare used to be, then not so much.
Nintendo already has franchises that are, quite frankly, considerably more popular and valuable in the long-term to Nintendo themselves: Mario, Zelda, Smash Bros., Mario Kart, Mario Party, Animal Crossing, Pokemon, Fire Emblem, Kirby, DK, Metroid, Paper Mario, etc. etc. etc.
There really is no way Nintendo would go along with that scenario, as they would have nothing to really gain, except theoretically Microsoft corporate backing, which they've done just fine without for decades. On the other hand, such a "merger" could very well do damage to the Nintendo brand.
So no. I think not. Especially with how well Switch has been selling thus far, while I'm still skeptical of the system and they need to show me more games I actually want to own for the thing before I ever buy one/ But I think that Nintendo is doing just fine on their own.