I think a character death is very likely within the next season or so, since I'm getting super Anakin vibes from Ezra with his whole "I need to get stronger to protect my friends" thing (and we all saw how that turned out for Anakin in the end). That said, I think Kanan is probably in the hotseat, for a number of reasons.
Firstly, his death would be the most impactful - he's effectively the leader of the crew and Ezra's master, and I can see his death being the catalyst that drives Ezra towards the dark side or away from the rebellion for good.
Secondly, he's the only Force-wielder in the show who can actually be considered a Jedi, and although Dave Filoni has used the justification of Ahsoka not being a Jedi to explain how she could live through the events of the OT while still keeping Yoda's deathbed speech intact, I still think he's going to want to be as respectful as possible towards George Lucas' original intentions with Yoda's lines in ROTJ, which means Kanan probably won't be running around for much longer (I don't see Ezra completing his training).
Thirdly, I feel as if Kanan's overarching character arc is beginning to come to a close. We are introduced to him as the "cowboy Jedi", who drinks and gets into bar fights and tries to ignore his past. Then he meets Ezra and attempts to turn over a new leaf and train him as a Padawan, but consistently grapples with his insecurities as a Jedi. Then he is officially knighted and after coming to terms with his blindness and failure, is now starting to move towards a true sense of Jedi enlightenment and peace with the Force. I feel like close to this would be a perfect time to kill Kanan, as he is at peace with himself and what he has become, and all things considered it would be a satisfying (if sad) conclusion to his character arc in my opinion.
With all that said though we know Dave Filoni hates killing off major characters so I dunno. Ezra won't be going anywhere any time soon, Zeb and Chopper are basically comic relief (and at its core Rebels is a kids' show after all), Sabine's character arc has a looooong way to go in my opinion, and Hera... eh. On the one hand I can see Hera dying as a complete curveball, but on the other hand I think they'll probably keep her around in canon for a good few in-universe years, maybe throw in a few references to her activities post-ROTJ for some fun shoutouts. Rex is a very likely candidate, and easy contender for saddest death in Star Wars. I can tell you that now, no matter how the old geezer makes his exit.
And we all know Chopper survives the events of the show because we see him as the Supreme Leader of the First Order in The Force Awakens, so he's not going anywhere.