Cassian (and Saw Gerrera) definitely show a darker side of the rebellion but Cassian at least starts to see a different path than just aping the empires follow orders doctrine.
Im not sure Vader is an example of a playable imperial. Last minute redemption is a great story point but up to that point he’s a vicious sadist.
The problem with the fiction is that I don’t see how you can play a ‘heroic’ or ‘good’ imperial for long given the Empires nature. It’s a repressive, facist civilisation that uses brute force, fear and immoral practices to sustain itself.
Now it’s a RPG and people might want to explore the monster within but I’m not built that way personally. For me an Imperial sourcebook is essentially a monster manual of bad guys and gear to beat on, blow up or steal.
I agree with you and several others in that I can’t see the House of Mouse exploring the darker side of the SW club.