Well, my character, who is a duke in the CE is quite proud and at times arrogant, and would love nothing more than to be a ruler of his own realm (although I myself do not particularly fancy it). But at the same time, he is also a staunch loyalist. There is really no motivation for a Duke of the empire to secede. Least of all while the war continues.
Ambitions? Being a Duke is almost the highest position possible, with only ruler topping it. And even then, it would be ruler of a duchy (which they have already) as opposed to one of five Dukes, and one of nine council members in one of the most powerful realms in Atamara.
Then, of course, there is the fact that the rulership of said realm and duchy would likely be lost in the ensuing conflict. There is currently no likely action that could get such a divergent reaction, short of banning a Duke (as far as I reckon it).
It sounds to me like the way for the Dukes to get what they want is to make a pact to all secede at once, and form an Imperial Council, which would consist of the dukes themselves, and could continue the running of the Empire without each Duke having to answer to someone else within his own Duchy (a Duchy which would become a sovereign realm unto itself).
This would, of course, screw the Duke of the capital duchy pretty badly, as he'd be the only one left under a separate Ruler. Unless the Duke of the capital city's duchy is, in fact, the ruler; I know little about CE, I'm afraid, or I could propose something more closely tailored
