I like the idea, but it would be easy enough to come up with a Warlock deck that has no way of dealing direct damage to the enemy hero. Your opponent would not be physically able to win, unless you conceded. Perhaps if both hero powers were Unstable Portal, like in the Rafaam boss battle.
That's why it should be singleton. Even in Warlock you probably can't find 30 cards that do not deal damage.
Maybe some cards that do nothing could be banned (like Sacrifice a demon)
--Alfi--
While I had lunch today I had some time to think and I thought up a possible funny idea for a brawl - Brawl of the saboteurs.
Basically, both players create a highlander deck (one card of each max). Then, they swap these decks and play who wins.
So you have to build the worst deck possible for your opponent and then beat it with the worst deck he could make.
What do you think? A real test of skill, no?
--Alfi--