Helpful party summary:
Fighter in a can, our one and only tank.
Shaman, our only current healer. Ranged heals and 'melee' healing checks, with Spirit Bear Companion being mobile offensive force (that interacts strangely with rules).
Warlock, ranged damage with some control.
Last session, we added one permanent character - Wizard. Ranged control with some damage.
We also had a guest Rogue, who apparently is not coming back, and our Invoker has been MIA for a rather long time.
We need a meatshield of some sort, either from our fighter in a can or a new fighter. After that, any role could be useful, but some classes fit with our current makeup better than others.
Here's my personal, Ranged Damage perspective: A class that lets the group as a whole do more damage (either by attacking or by letting the two ranged damage hit harder / more often) would be helpful. A class that can pin down or herd enemies, to keep them from hurting us and shutting us down, would also be helpful. A class that can do both would be ideal.