I think the game that has done it the best thus far has been City of Heroes/villains. Although you certainly still have tank, dps, and support classes, there is plenty of overlap so that any team of 8 can be viable. You don't even really have a pure healer at all, although there are heals. Buffs and debuffs are equally important and can fully replace a lack of heals. Likewise, you don't need a tank to soak the alpha, a scrapper, brute, or even a mastermind can as well, or you can have a controller or dominator mezz the mobs and prevent the alpha from occurring at all.