Bartholemew, then. He should have had a bigger role in S4, there is a story of him either in the newspaper or books that he sabotaged a Kooluk ship or something, so it would have been awesome if we recruited him on a mission, rather than just picking him up in a house in the Nay kobold village. At the end of 4 his epilogue says that he travels to Falena, so it would be interesting to know what else he got up to, and who might have been the ruling family members of Falena at the time.
I would also like to know more about Eresh. Or another sorceress such as Hellion or Maxine. Jeane and Viki obviously have their secrets and mysteries, but I got the impression they were going to be dragged on all the way through the series.