With the Woldwrath coming, any attack spell's stock is going to go way up especially anything with a crit effect.
However, the spell I would most like to see on a warlock's list is Fog of War. Stealthed Stalkers, Groves, Morraig wherever they want to be, camouflaged druids and reeves and naturally high DEF on most everything else. It's powerful to be sure, but not really any more so than some of the toys the other factions are playing with, and it fits Circle very well. Perhaps balance it by having it on a low FURY 6 or even our first FURY 5 warlock.
What spells would you guys like to see?