What about a custom game option where each team bans a champion class?

A Summoners Rift mode. All champions unlocked. Both teams can ban all Marksmen/Tanks/Assassins/Mages/Bruisers which will leave any combination of 3 remaining classes for players to choose from.

OR

Each team votes for a class their team must draft from exclusively. So an all assassin team vs all tank, or all marksman vs all mage etc.

I've had ranked games that happened this way by coincidence and they were really interesting to play out. I don't see this as a party mode like URF or OFA or anything. Might be cool as an option for custom games though.