Even though the Force Unleashed games are no longer canon, and therefore Galen doesn’t currently exist in the same continuity as Ahsoka and the other Mandoverse shows, that certainly doesn’t mean he couldn’t be reintroduced in the new timeline with some tweaks. Witwer himself said in a 2017 interview that Ahsoka showrunner Dave Filoni had considered bringing Starkiller back as an Inquisitor for the Rebels animated, but that it just didn’t fit the story Filoni was telling at the time. In Ahsoka, however, an Inquisitor Starkiller makes a bit of sense.
Yes, Ahsoka is about the search for Ezra Bridger and Grand Admiral Thrawn, but it’s also thematically about the relationship between master and apprentice. Since Starkiller and Ahsoka are both apprentices of Anakin Skywalker (albeit in different continuities and contexts), it would be fitting to see their paths intersect in the series. The series is also very interested in exploring unconventional Force-sensitive characters who aren’t exactly Jedi — Ahsoka, Baylan, Sabine, Ezra, and Morgan all fit the bill — and Galen Marek would certainly work as yet another perspective into a Force wielder who exists beyond the Jedi and the Sith.
But while Witwer’s Galen Marek is the most prevailing theory regarding Marrok’s identity, there are others who believe that the villain is actually Ezra turned to the dark side. A lot of support for this theory comes from the Arthurian legend surrounding the name Marrok. According to legend, Marrok was a knight who was controlled by a witch with connections to Morgan le Fay, and in some texts is even referred to as a Werewolf Knight. Given Morgan Elsbeth’s connections to the witchy Nightsisters of Dathomir, Ezra’s connection to the Loth-wolves, and Ezra’s brief dalliances with the dark side in Rebels this doesn’t seem entirely outside of the realm of possibility.
However, given that Thrawn and Ezra disappeared together at the end of Rebels, and we know that Thrawn is currently in another galaxy, it seems unlikely that Ezra is terrorizing our heroes and not trapped with Thrawn in Peridia beyond known space. Could they have been separated at some point in the decade they’ve been missing? Possibly! But things don’t quite line up as cleanly for this theory.
Plus, it has to be said that the names “Marek” and “Marrok” are too alike to simply be a coincidence, right?