About the "playing both sides" part. Maybe give the leader the ability to control the armies? He effectively becomes Supreme Commander and has total control over the more noble nobles' armies (as the corrupt ones wouldnt let anyone 'steal' their control). Maybe even add a casus belli mechanic, wars can be against the enemy leader (suspected of enthralled), or just for taking a duchy. Failure to succeed means huge loss of prestige, so a lost war before elections could mean deposition. This also allows a player controlled leader to intentionally lose the war against the dark empire, ( as the enemy leader hasnt been deposed in X turns) but would mean he gets deposed, so a one time trick to give you an advantage. Though, maybe give a dark vs light empire war a special flavor, make it last a long time and never end unless one faction wins.
Viewing post in V17 Beta, Discussion about End-Game comments
Casus Belli mechanics should probably be implemented sometime, along with occupied locations, so the territory only changes hands at the end of war between human forces. Probably doesn't need to be as complex as CKII/CKIII/Stellaris' approaches, but it doesn't seem too historically accurate for an entire empire to consume another in a giant war. And having cities only switch once might help a bit with nobles maintaining their positions despite their cities being invaded.
Not sure it can make it for the next version, as we're already going to try to add so much, but it would definitely make combat a bit better. Certainly there should be a "crusade" mechanic, because current if a nation is afraid of dark nobles they'll invade their country, but then give those very same dark nobles a vote in whether or not to pronounce them guilty. It means that invading a dark nation can suddenly just add a bunch of dark nobles to your population who then doom your nation, despite people being aware of them.
I also like the idea of reputation being 'at stake' from taking actions. We tried for a bit to have every vote be proposed by a person, whose prestige would change if the vote passed/failed, but it just made stuff more complex. Having the leader's prestige be affected by war outcomes could help.
I was also thinking of an agent who would benefit from deliberately losing wars against human nations. Will keep it as a surprise, but I think it could be fun, at least. Perhaps not very balanced, but an amusing way to play, and a game should probably try to be fun, not just hard.