Gnome Warlock or Mage, Dwarf Priest for Alliance
or UD for all 3 if Horde.
Warlocks are rather beast however. Being able to handle mass pulls with Voidwalker as you level until you hit 60. At that point you’ll want to use Succubus for Charm/WPvP when in the open and just start nuking targets down with impunity. You will need to be careful though because Rogues and Warriors will be absolutely nasty when they close the gap (and they almost always will) but you should be able to destroy every other class with little issue if played well, though UD will be difficult to Charm/Soulfire nuke. But that is just one build, there are plenty of options for a warlock to ruin someones day.
Also if Blizzard doesn’t include Meeting Stones (which they shouldn’t) then as a warlock you will be wanted to get groups rolling for dungeons. But on a PvP server you will be wanted anyways because the Meeting Stone at Blackrock would be a warzone when not camped by rogues.
Soulstone is an invaluable tool for wipe recovery (or even wipe prevention during boss attempts) and is incredibly useful along with healthstone healing for important raid/party members.
Also when it comes to PvE, unlike PvP, a warlock is pretty straightforward. Just go Sac/Ruin, CoS/E and hit ShadowBolt till the boss falls down. Yeah SM/Ruin is a thing but I’m lazy and I don’t like my Imp.