There is a way to do tag team, Google "tag team workshop" or something and you'll see. I've used it for Cosmi vs. eGames. But if you mean it with three character tag team like in MvC2&3, I don't know. Stage interactions are possible with zoffsets or (stagevar triggers for 1.x). I do it for breakable objects, ambient sound and low health music. So, what I would like in M.U.G.E.N are as follows:
Continue Music & Timer
Score System
New Challenger (other player can join during match)
Character-specific victory themes at the victory screen (a la Marvel Super Heroes and Darkstalkers)
Animated Character Icons
Scrolling Character Portraits at VS Screen
Next Opponent (and Stage) Screen
Use Player 3/4's win quote & portrait if either of their team wins with Player 1/2 dead