Rick isn't likely to ever be killed. It's possible, but highly unlikely (possibly as a series finale). It'd be a huge shift from the longstanding centralized, main character narrative and Fear the Walking Dead has seemingly already distinguished itself from The Walking Dead by diffusing the character leads. They'd likely have to set up a new lead character that contained a natural storyline progression, like Carl or even Judith if it either ran long enough or they jumped ahead in time following his death.
Rick is the only one I feel is safe from death (not from injury though).
As for the season finale, I read a good point somewhere that it isn't likely to be Carl due to Negan's threat to his men regarding him as he kills one of the group members. So it would make sense that Carl is not the one being killed if he's being used as leverage against the rest of the group.
In the comics, it's supposed to be Glenn. As Glenn is one of the longest surviving members (Rick, Carl, Carol, Daryl, Glenn, and Morgan have survived since the premiere although Morgan wasn't a central cast member until this season) it might be seen as a letdown if it's one of the comparably lesser characters like Eugene, Aaron, Sasha, or Rosita. I get the feeling it may be Eugene due to his exchange with Rick earlier in the episode, but it may be any of them other than Rick or Carl. Before the start of the episode, I was getting the sense that it may have been leading towards Maggie due to her pregnancy. Her relationship with Glenn might be construed as a nod to the outcome in the comics, but also because of her pregnancy the show might be disinclined to have another baby in the story. It's difficult to write and rationalize that in the storyline while also having the characters acknowledge the difficulty. Given the events in the final episode though, she could survive and still lose the child. The death of Abraham, Sasha, or Rosita would be another aspect in their strange relationship triangle they've been going through, but I think it would still be a very weak development.