Adding to Colton Vickery's comment, I have my own suspicions. My gut says Burt, but I had no knowledge of his continued inclusion (plus he already had a heart attack), so I've eliminated him. Carl was my next guess, but then I thought he wouldn't affect ND or any other group (unless they cry over losing their dentist); also, Stamos apparently is confirmed for 10 episodes, and he's nowhere close to that.
I agree with Blaine, Kurt, Rachel, Finn, and Quinn all being accounted for based on the posts I've seen. April hasn't been around yet, I know it's not her. Quinn's mother wasn't brought up in that answer, but would they bring her back just to kill her? Does that scene happen after they perform...or before? I doubt it's a ND member for one reason: they have 12 people in the group now, and they apparently go before Dalton Academy; if one of them dies, they have 11 and are DQed since Kurt performs with Dalton before going back to McKinley later in the episode (apparently taking Blaine with him when the Warblers are finished, ND apparently wins Regionals too). Thus, Tina is safe in my books as well as everyone else, but having her missing from the previews is ominous. Watch for the trailer on March 8 after "Sexy" to see if she appears briefly and we can revisit this.
Let's cut to Colton's short list. Figgins, Will, Emma, Beiste, Brad (ND's pianist), Carole. Tina, and Jacob. In reverse order: Jacob: nope, wouldn't affect ND. Tina: possible, but again would DQ ND, so no. Carole: also possible, and a good candidate. Brad: not a storyline character, replaceable and wouldn't affect the team that much. Beiste: maybe. Emma I will eliminate; Stamos has 10 episodes confirmed, and without Emma in the show would not have any role I can see. Will: no way, he is literally THE main character, and the only possibility if he goes is that Paltrow would take over Glee Club permanently; I do not believe this has even been suggested in the furthest rumours. Figgins; not a major character, but nonetheless possible as the entire school would be affected by it.
Therefore, my short list: Figgins, Beiste, and Carole. Beiste doesn't seem as likely to be a big effect. Figgins could end up going, and a new principal comes in, becomes aware of something about the football team, and someone gets expelled. This is a possible storyline, but Carole would have a deep effect on both Finn and Kurt (his new step-mother) if she dies off, so I would not at all be shocked to see her in her deathbed. EDIT: Now, apparently Carole's actress has recently canceled other commitments to appear on Glee, so I have to now count her out. Beiste seems like the only plausible person to kill off at the moment, as they won't drop Figgins I think (though it's still possible), and the bird is still a candidate.
Side note: Artie was left off the previous post, also; it's a possibility, but again would disqualify ND; was he pictured? I wonder if Jacob has to come in to fake-dance again; if so, that puts Artie and Tina back in the short list.