I want to be clear; while I went hard sayin Syril Karn's possibly my favorite character, it's absolutely because of his flaws. I'm not suggesting he's a great dude. What I find so interesting is how easily those characteristics would also make him a fantastic traitor and rebel. He's driven by a sense of justice; he won't let Cassian Andor drop because the man committed a double murder under his watch, and he knows it. He doesn't even particularly need the spotlight; he wants to be respected but he's not driven by a pursuit of fame. He's absolutely willing to break all the rules and ignore direct orders in his pursuit of that "justice"; he misuses his authorities in the Bureau of Standards to keep pursuing Andor, and refuses to drop it even when Imperial Intelligence orders him to. The moment Karn no longer thinks the Empire is the bastion of justice, he'll flip. He'll agonize over it, but he'll flip. That potential's already seeded, and he's nowhere close to that at the end of Season 1, but by the end of Season 2, which is apparently supposed to take place over 3-4 years of time?
I don't think he'll ever forgive Andor, but I think he could be convinced to work alongside Andor as long as he knows Andor isn't gonna survive (Rogue One's mission), or if Andor or his leadership have agreed that Andor will turn himself in once they achieve a goal he's needed for.
We've already seen Imperial Intelligence doing shady unjust shit. If Karn learns of that standard practice through a budding romance with Dedra, I don't think he'd stay loyal, if he's convinced it's true. Whether the two of them betray together or it's the flexion point that shatters their relationship (potentially leading to Dedra absolutely rabidly hunting him down, a nice bookend of her initial rejection of Karn), there's a good story there, IMO, a lot of nuance and "gray". We know Imperials do get convinced to rebel, we've already seen one in Andor, Karn could be the guy we follow through making that choice.