Yeah obviously it could be Ezra, Maul (former Sith apprentice), Kanan (former Jedi apprentice) or Ahsoka (former Jedi apprentice).
Ezra not being Ahsoka's apprentice seems like it would be a missed opportunity. And given there are some parallels between Ezra and Anakin it would be interesting for Ahsoka in an Obi-Wan role to have deal with that and ensure Ezra doesn't fall ike Anakin. Also Ahsoka and Vader can have another showdown later in or at the end of the series. It doesn't have to be finished this season.
Kanan
OT but to speculate on how fully trained and developed Ahsoka, Kanan and Ezra would have likely fit into the prequel era Jedi Order; I don't see a fully trained Kanan being more than a Jedi Knight, while Ahsoka and Ezra not only would have become Jedi Masters, but probably even Jedi Council members
As that being a surprise twist to the season. I agree 100% with your theory.
In fairness to Kanan he never got to complete training. I don't think Ahsoka is a natural beast but she had some much experience during the CW. Ezra, as they are writing and developing him would been top tier.
In fairness to Kanan he never got to complete training. I don't think Ahsoka is a natural beast but she had some much experience during the CW. Ezra, as they are writing and developing him would been top tier.
Anakin 
Vader 
