Hmm... how about an ADOMPossible style game, but rather than doing steals like described in the link Soirana lists, the lowest scoring team(s) can inflict restrictions on PCs from the highest scoring teams in future missions (chosen from a list or something so it doesn't get too obscene)? So if Team A has a wizard that is doing very well, the losing Team B could add a restriction that for the next mission, that PC can't cast spells, say.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.