There is already a lot of magical power in the group (two rogue/wizards, a cleric/warlock, and a human paragon/wizard), as well as an urban ranger and a fighter. There is also an NPC cleric/fighter who will be backup when needed.
The group doesn't have a pure cleric or rogue and @Tellerian Hawke's half-ogre PC is the only pure fighter in the group. (Right now, @JustinCase's kobold urban ranger is his frontline backup, although Custodio [the NPC] could work up front too.)
So, I'd say go cleric, rogue or fighter. Hmm, paladin?
Most of the alignments are lawful and/or neutral with one CG character and the NPC who is NG. Note that I prefer rogues to not be LG and rangers to be partially neutral for World of Kulan.
The group has an Urban Ranger so why not an Urban Druid, if you have access to the DRAGON Compendium hardcover. And you can pick just about any class from UA.