I'm interested, but I want to see a solid plot of some kind first. It'll be difficult to replicate Brawls, since they often occur between multiple people fighting each other all at once. Unless players could perform only one attack per turn and must state who they are attacking, and if they are being attacked. We could also make HP a fraction to avoid random percentages KOing other people (which often happens in Brawl)

You could set it up so that each player can either attack or defend on their turn, depending on the actions of other players. 

Player:
Attack: They attack a player
Defense: They attempt defend against an attack

Response to Player:
Attack: Attacks coming at them from another Player
Defense: Result of their attempt to defend

Items: GM states that an item has entered the arena

Note that Attack and Defense correlate to each other. Example-
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(Mario):
You respawn and are invincible for one turn. No actions
Attack: none
Defense: none

(Link):
Attack: I shoot an arrow at Bowser when he respawns
Defense: none

(Bowser):
You respawn and are invincible for one turn. No actions
Attack: none
Defense: none

(Response to Mario):
Attack: none (no attacks are coming at him)
Defense: none (nothing to defend)

(Response to Link):
Attack: none (no attacks are coming at him)
Defense: none (nothing to defend)

(Response to Bowser):
Attack: Link fires an arrow at you (Attack from Player's turn)
Defense: none (no previous attempt to defend against an attack)

(Mario): 
Attack: I shoot a fireball at Link
Defense: none (nothing to defend still)

(Link):
Attack: I throw a bomb at Mario
Defense: none (nothing to defend still)

(Bowser):
Attack: none
Defense: I attempt to jump over the arrow (Defend against Attack)

(Response to Mario): 
Attack: Link throws a bomb at you 
Defense: none (no previous chance to defend)

(Response to Link):
Attack: Mario shoots a fireball at you
Defense: none (no previous chance to defend)

(Response to Bowser)
Attack: none
Defense: You jump over the arrow for 0 DMG (defense successful)

Items: Sticky Bomb enters the arena (posted by GM)

On their next turn, Link can attempt to block the fireball (or attack and take damage or run for the item and take damage), Mario will attempt to run away from the bomb (or attack and take damage or run for the item and take damage) and Bowser will perform an attack (or run for the item or do nothing).

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This is going to be a lot of work if that's how you want to play it.