Basically a reverse ruleset, where stocks lost per character are not counted, each match starts with 4 stocks (or whichever other number is chosen) and instead the win condition is to score a match win with all the characters picked by the player to be declared the overall winner.
Winning a match with a character would make the winner unable to use that character for the rest of the set and have that character counted as a character that a match has been won with; loser would continue to be able to pick their character as usual for the next matches.
This could potentially be a separate mode in the menu and not strictly require 12 characters to be chosen for each player.