Tag team brawl:
You select 2 characters and are able to switch between them in game (you would be able to choose how long it took, example: instantaneous or the transformation time of zelda/sheik) then you would be able to fight as two separate characters in the same match, adding greater strategy (like picking speed and power, or your main with a recovery character like peach or jigglypuff, or maybe some cheap combos, like smashing someone into the ground with DK only to swithch to jigglypuff to kill them with downB the instant they get out). Also, you would get the option of wheather or not the characters shared health (example of not sharing health: "shoot, my marth has high damage, better switch to gannondorf, who has a lower hit %, example of sharing health: "I'm at high damage and it won't change if I switch characters") There would also be an option of wheather one character dying would reset the other's damage.
Link brawl:
You choose 2 characters, a main one and a secondary one, and play both at once like ice climbers
Elevator brawl:
Kind of like low and high gravity brawls combined...The stage would move up rapidly (causing a high-gravity effect) and down rapidly (causing a low-gravity, free-fall effect) at random intervals
One-sided brawl:
You can only KO opponents by knocking them off (a) certain side(s) of the stage (top, bottom, left, right). If they hit any other boundry, then they would bounce taking damage (like they hit lava on Samus's stage). You would be able to decide which side(s) a player could by knocked off, or make the side(s) change randomly.