For all my predictions of Luna and Applejack, I honestly couldn't believe it when I read that they're killing off a major character we know and love due to merchandising issues and a need to cut some budgets. I thought Jayson was joking when he said somepony would die. I hope this is a joke too, but if not, we might lose one of the Mane 6, or one of the princesses.
I do not think they'd kill off Spike or the Cutie Mark Crusaders as it would be too dark even for what we're talking here. I'd think either Celestia or Discord, since they should keep all the Mane 6 around and the fandom will riot if Luna is killed. But in response to a "if you had to choose one" question, a lot of people are saying Rainbow Dash. Some say Diamond Tiara, but they said "know and love", and the "love" part doesn't fit with her.
There is also possibility that it could be Applejack, the supposed background pony, or Rarity, who didn't get much love from the writers in Season 3.
But I think the three most likely to die are Celestia, Discord, and Rainbow Dash. I will say this: I could accept them killing off Celestia or Discord, though I definitely don't have to like it (especially Discord, seeing how fun he is to have around but I'm not too worried about him, he survived decapitation). But if they kill Luna or any of the Mane 6, I'll FanonDiscontinuity Season 5, just like if it said Chrysalis survived. Though I'm sure they wouldn't be so stupid as to kill Luna. I know I've had some evil premonitions of her before but now I am confident that she's staying to the end of the series.