If you want to do some 2-box instances later, then try to get Tank-Healer-DPS abilities into 2 classes.

Let's start with a tank.

Warrior. Tank/DPS
Paladin. Tank/Healer or Tank/DPS (more Prot Spec, and rest in Ret tree).
Druid. Tank/Healer or Tank/DPS
I suppose the other tank you can use is pet, Hunter or Warlock.

Healer.
Priest - Holy/Shadow does not seem to mix well. Holy / Smite is viable.
Druid - Healer / DPS, or Healer / Tank
Shaman - Resto / DPS
Paladin - Holy with some DPS Sanctity Aura and Holy Shock build, or Holy with some Prot.