Why do you want this add-on?
Portrait battles are fun, but most are rather dull, the arena would put all portrait battles of a mansion in a random order for you to complete (with a few static finalists)

Describe the add-on you'd like in as much detail as possible
Simply put: Kirby games' (True) Arena

Additional context
In ye ol' map variant, there was an arena mode; this add-on would bring it back (just not the additional bosses)