Some people are saying it's Karofsky или 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 сказал(-а) 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.