Your group is already really good as is. Everything considered, there's three or four possible choices out there:

A) Divine caster. Obvious.

B) Factotum. Limited Heals, and if you take FoI, you're awesome all of the time.

C) Binder. "Hey, did anybody need anything short of an arcanist?"

D) Wizard. Never have enough. Sorcerer if you want to feel pimp because you use Solo's Supah S guide.

E) Swordsage. Your heals will suffer a bit if your beguiler doesn't get a few wands of lesser vigor, but you're going to kill everything reeeeeeeeeeeaaaaaaaallllllly fast.