Played with a few formats over the years. Couple of popular ones as examples:

5 person ala WoW: 1 Tank, 1 Healer, 3 dps
6 person ala Aion: 1 Tank. 1 Healer, 4 dps - or one of the dps could be a support class (Chanter for example) - which was the intended ideal group at launch.

Aion does have the option of letting the group organizer decide what they want in the group. It is completely flexible and doesn't dictate which class roles should be in it.
I think this works very well, but of course would require a massive amount of dungeon re-work for it to appear here.