What would make a Tag Team mode really awesome (IMO) would be:
Tag in moves- When you tag in your teammate, before you fully switch, they perform a tag team move based on which way you flick the stick: Up, Down, Left, Right.
Tag Team Fatalities- You can perform solo fatalities still, but you'd have the option to do a TT one.
Kombo Team- This is a Kombo that you use to perform a move with your partner, but won't switch out.
Also there can be multiple Tag Teams (ie. Liu Kang & Raiden, LK & KL...etc) with other character and have special moves together, just not everyone Tags cohesively (ie LK & Kano) with each other. Where they would have generic Tag Team moves if that makes any sense
How does that sound?