Some people are saying it's Karofsky of Burt Hummel. I don't think it's Karofsky (they developed his character too much to kill him off) and, whilst it might be Burt, I doubt they'd do that. :/
If there is a death, it would not be a main character's. They have zei it would not be Burt Hummel because Mike O'Malley is staying on the show. Some say it may be Karofsky, but it does not quite make any sense yet.