Sorry, but the fan speculation about who Marrok the Inquisitor really is has gotten completely out of hand, as Star Wars fan speculation is wont to do.
No—it’s not Ezra. If Morgan et al. had found Ezra already, they would have found Thrawn as well. And no—it’s not Starkiller, either; there is no way they’re going to pull some character from a game like that and put him in the show.
I saw somewhere the other day where some YouTube rando was convinced that it is the clone of Luke Skywalker, made from the hand he lost at Cloud City. That is when I decided that I really hope Marrok is no one we have ever heard of, just some random Inquisitor.