I'm also planning on creating a mixed 5 box team soon. I am wrapping up my level 80 DK + 4x elem shaman group and want some more variety for my next team.
I am planning on the following group which I think will be flexible and have tons of buffs and mixed armor type requirements.

Paladin (tank, buffs, rez, plate armor)
Mage (dps, portals, water, food, buffs, cloth dps armor)
Shaman (dps, totems buffs, wipe recovery, rez, cloth, leather, or dps mail armor)
Druid (dps, buffs, rez, cloth or leather dps armor)
Priest (healer, buffs, rez, cloth healing armor)

Should be a fun and flexible group, and more interesting building since I now have experience 5 boxing heroic dungeons with my old group.