I think Rick and Darryl are untouchable, and probably Carl. They won't kill off any of them. Darryl became too big of a fan favorite, he's used to sell all there merchandise and games, and the Rick and Coarl story has more to go. I'm guessing Carol and Morgan have a longer story too, so their safe.
So out of the OGs, only Glenn is open to be killed off, but he's a fan favorite too.
All of the second group people, Abraham, etc, are open to die. Come to think of it, Abraham is liked enough to be a big shock but not big enough to be a huge loss so I'll say he's likely to be killed season finale or maybe early next season. I definitely think Glenn though, or Maggie...one of them has to go so the other will be in agony (good TV). She cut her hair, man, that's TV suicide lol