K2SO is really cool, but a whole show dedicated to this guy is a mistake. I have always felt the Cassian casting was sorta botched. I'm not necessarily saying he is a bad actor, but he just doesn't seem like a badass, do anything necessary spy.
I thought he was fine. In Rogue One he had to face the morally questionable stuff he'd been doing for the Rebellion (such as killing his contact to escape capture early on) and decide whether to keep doing them or trust JE about her father. I think it could be quite interesting to see the evolution of his character - i.e., what happened to get him to that point. And being a spy isn't always about being a badass - that usually draws attention to you, which is the last thing a spy wants.