Imoen would maybe be a good choice. If you needed to RP it, the justification could be you're leading her along solely for her skills - you know you need them, and its just that's all you can do. I personally like Jan, and I think he's underrated. In ToB, he gets all the stuff you would want out of a thief (*nudge nudge*), and he's a very passable auxiliary spellcaster, as well as being a decent archer. He's a pretty good support party member, since you have the main areas (Fighter, Mage) already covered.
For the last slot I would consider three alternatives: Haer'Dalis, Anomen and Jaheira. Haer'Dalis gets insanely powerful later on in the game, but I would go with a cleric for the spells. I would go with Anomen, because cleric extras are more useful for this party makeup than Jaheira's. And remember to...try to mold Anomen's personality.
