Hearthpwn

 Both players play as Rafaam, you play the deck that the other player has brought to the game. Hero power is the same as the adventure (unstable portal).

Rules for deckbuilding: The deck must contain 3 of each part of the mana curve (1 - 6, 7+), the other 9 cards can be any cost, so you can't totally cheese your opponent by putting in all 1/1s

I think this would be interesting and have huge replay value since you never know what you're going to get.

People would figure out the most useless deck and everyone will play mirror natches lol all1/1's is good.. all highcosts legendaries is worse.

What I did against rafaam heroic is a deck that only draws cards

Well, again, that's why the 3 of each part of the mana curve restriction is there, so the deck will be reasonably playable. I'm mean sure, you put in 3 of the crappiest 1 drops, 3 of the crappiest 2 drops, etc.. but it would still be interesting.

What about doing something like switching out half the cards of both players. The cards being swapped should be of same type & mana cost, or as close as possible.

This ensures that you'll always play with half of the deck yourself and it's much harder to cheese your opponent. Could be fun I think. Really like the idea of "random" yet predefined decks. It's one of the reasons I love playing Arena.