I wouldn't worry too much about "filling gaps" in the group as much as I would making a character that will be enjoyable for you to play. I mean, heck, one of the characters making up the current group is basically a non-combatant whose only useful traits seem to be the ability to universally understand languages and that he's generally a likable guy. Your character can basically be anything, as long as it adheres to the rules of the Prophecy universe. That said, if you do want to specifically fill a vacant "role" in the group, I suppose they're missing (a) mage(s).