People Marrok might actually be, because Ezra is an obvious red herring:
1. Barriss Offee - mentioned her before. She betrayed Ahsoka and framed her for a terrorist attack in Clone Wars. She was last seen being led away by Palpatine's men, so it would make sense that she would become an Inquisitor. Of course, the character is played by a male stuntman so that casts doubt.
2. A canon version of Starkiller from Force Unleashed - Starkiller's voice actor from the game, Sam Witwer, is credited as "additional voices" in all 3 episodes of Ahsoka so far. And the character's name is Marek - close enough to Marrok. The game isn't canon but neither is KOTOR, and they've selectively added stuff from KOTOR into the canon as they've gone along.
3. Future (or alternate universe) Jacen Syndulla - we know time travel exists due to the World Between Worlds (watch Rebels if you don't know, assbutts). In the old expanded universe, Jacen Solo was Han and Leia's son who fell to the dark side and became a sith lord. Jacen Syndulla is the son of General Hera Syndulla and Jedi Knight Kanan Jarrus, one of the other main characters from Rebels.
4. Petro (one of the young Jedi that Ahsoka mentored way back in the Clone Wars). Huyang, the robot that is on the ship with Sabine and Ahsoka, featured prominently in that arc as well. Petro was shown to be the most impulsive and confident of all the younglings, much like Anakin. It would be a good callback.
5. Just some random Inquisitor whose sole purpose is to get killed in this episode so it seems like it had stakes while the two dark Jedi whose faces we've seen get away to be dealt with in future episodes.