If you're not going with trinity, I'd suggest going with whatever gives maximum synergy.

If you're thinking a caster group, you'd want an elemental shaman as one of them for sure because of the totems. Boomkins, mages, and shadow priests give some nice buffs as well. I'm not sure what the best buff synergies are for melee groups. Is it feral druid, enhance shammy, and DK/paladin? I've never played any melee characters except druids so I don't exactly know.