Just to clear up your "Nah" list, it isn't Glenn because he's a comic character. Lee and anyone else in that group will NEVER interact with him again. Ever.
The radio guy is going to be someone that we as gamers haven't encountered before. It might be someone from Clem or Lee's past, but we haven't interacted with them before. We could speculate all day and probably still never come up with who it is. Everyone else that our characters have encountered is either dead (including people who have a CHANCE to be dead like both St. Johns, Andre the police officer in Clem's neighborhood, Chet etc.) or comic characters who will never appear again (Lilly as she was originally intended to be, Glenn, Hershel)
It seems unlikely that it's anyone with a personal grudge against Lee though. It just wouldn't make sense or be plausible. The entire world ended, but this person hates Lee enough to track him all these months, see how Clementine was so important to him, some how pick up on the fact that they were heading to Savannah and get there to steal Clem away?
I'm guessing it's just gonna be some nut job. Probably not even connected to anyone by any means.