This is the one thing I would love to see more than anything else.
There can be two modes: 1) War to the Death with full loot and permadeath of characters. You can only win the most awesome stuff from other players by risking it all. 2) Arena where each player is given a random drawn party of characters and gear to assemble before battle kind of like hearthstone’s system. Or a base of crowns to purchase and hire an optimal party from a standard selection available to both players.
As for 2v2s, FFAs, 5v5s, coop multiplayer etc…. A man can dream :)