Buuuhhh! That is a hilarious ability. If all players remaining are teammates, do you all automatically win the game?

Oh, I sort of forgot about that element. I guess the game would end in a draw if there were only two players.

Hmm. But normally if all remaining players are teammates, that team wins the game, not draws. Eg in a 2HG game or 3v3 Emperor if you eliminate the opposing team. You don't draw, you win!

But if you both win simultaneously, in a tournament setting, isn't that a draw?

In a tournament, yes. Which may be somewhat counterintuitive, but I guess it's no odder than an Earthquake making all players lose at the same time leading to a draw.

Earthquake for a draw generally costs more than 6. Divine Intervention, which had that as its sole, specific purpose, costs 8 and two turns.

­Slithery Stalker was changed in development to make it unable to target creatures you control, because the then-combo with Shifting Sky, for a total cost of 6 mana and two cards, was thought of as too easy a hurdle to allow draws out of nowhere.

Frenemies? It does make me wonder if this card should say "Target opponent is now your teammate. They are still your opponent."? Also, is this one sided? Can player A be player B's teammate, but player B be player A's opponent?

The CR treats "teammate" and "opponent" as strict antonyms, so what would an ability like that even mean? "2+2 is 5, but Big Brother says it's still 4 too."

Well, I can consider you my friend, while you consider me your enemy. But, no, I'm pretty sure the rules don't work that way. I'm just throwing out suggestions.