If they ever make a return in the future, then I have some ideas for them.

When a character gets uppercutted on a upper floor, there could be a gameplay feature that sees the screen split in two: The upper screen shows the opponent on the floor above, the lower one shows the other opponent. The gameplay itself would change, with a different control scheme.

Not every characters should be able to jump on the higher floor just like that. Some characters should be forced to try and get there. Let's say you're fighting in the Bank and your opponent ends up on the rooftop. Some characters would have to use the stairs. Your opponent can try to stop you from reaching the roof with projectiles (if he/she got any) or shaking attacks like Jax's groundpound. You would have the ability to dodge these attacks, with some characters being able to do it their own way. If the fighter above doesn't have any projectiles or shaking moves, two things can happen: he/she can go back to the lower floor, fighting the opponent along the way which could result in one or both of them falling down the stairs. Or he/she can try to ambush the enemy.

When that happens, one of the screens will disappear. When playing alone, or when playing online with you being on the lower floor the disappearing screen will be your opponent's (duh), or else it wouldn't really be an ambush now, would it? As for offline multiplayer, I'm thinking this whole feature would be turned off for it, with the player on the lower floor jumping/teleporting on the upper one right away.

Like I said in the second paragraph, not every characters would have to try and reach the upper level. Some, like Kintaro and Sheeva, could just jump through the hole. Those with teleporting abilities like Raiden and Motaro could do just that, teleport right up there. Kano could propel himself in the air with an upper cannonball.

To conclude, I'm not actually done elaborating my ideas, but it would be too much for just one post.