I'd love a play mode where I could play as one of the boss-mobs in the game, and get to play the crazy-powerful cards that you never get to use normally. Who doesn't want to play all those cards that only dragons and giants get? It could be a bridge-mode between PVP and Quest play for players who've graduated the campaign. Players in PVP would click on a new icon for "Be the Dungeon Master". Or the Evil Overlord. Or whatever. Players in the Campaign would see "Versus a DM" as a new Quest option. I'd imagine Blue Manchu doing this as a special event first. That gives them a chance to try it out, take it back offline and rebalance as needed. Or to can it if it turns out to drain too many players off of multiplayer. Maybe it might only be turned on for a few hours a day, as a special treat. However it was handled, it would be super fun to get to play "the bad guys" once in a while.
You can already do this in Custom scenarios, sort of. If you invite a friend you can pretty much DM for him - while controlling the mobs of your (or a mapmakers) choice. Also, there's already some player created maps in the game, but these are SP only (Mauve Manticore series). http://forums.cardhunter.com/forums/custom-scenarios-and-maps.12/
Well if you use the scenario editor you can create your own games (to play in MP or against the AI) using any monsters in the game. If you don't want to take the time to build a board just go to the custom scenarios thread and download a board that you like that someone has posted and then change the monsters to your liking after you load it as a custom game. EDIT: Penguin answered while I was typing... so basically I 2nd his thoughts.
I have to say, I do like the idea of people queuing up to DM and other people queuing up to play in player-DM'd modules. But we'd probably need a lot more active players before that scene would become lively enough for the feature to always be "on."
That's also possible (more or less) using custom scenarios: just uncheck the "Is AI" box for the opponent.