If they kill off Walter, they might as well just cancel the show anyway.
Speculation, not spoiler, but what the hey, everyone else is spoiler boxing them, so when in Rome
From a story standpoint Peter makes sense, he finds a way to get into the machine and somehow save both 'verses, but from a “show wouldn’t be the same without him” standpoint, I hope it’s Astrid or Broyles (assuming it’s someone from the blue team). They’re great and all, but killing either off wouldn’t really mess up the dynamic as much as Walt, Pete, or Liv. Of course, there’s always Nina, but I don’t really see her as part of the team anyway. She always seems to have her own agenda.