Well, given Harry's status or lack thereof with the White Counsel, it's got to be someone already estranged from the White Counsel and/or has no interest or hope of getting in their good graces.
I also think that Butcher is unlikely to intro a major new character at this point.
Looking back, I'm going with Fitz from Ghost Story. Certainly seemed like that character was going somewhere. Took up a subplot and a lot of ink but then exited midway thru that book, and haven't seen him since. Fitz felt like groundwork, and you know how Butcher drops stuff in books from left field and then uses the stuff later.
There was some young girl that showed some talent - maybe in one of the short stories, don't remember her name - but I don't think Butcher is likely to give Dresden another female apprentice, another female with magical/supernatural abilities, or another female in his life - with Mab, Molly, Maggie, Bonnie, Laura, etc. Dresden has enough female bosses/allies/friendlies in his life he has to deal with basically every day. Also, with Thomas out of a picture for a while, Dresden needs a replacement male for some balance.