The relationship of the Duchy to the Sparks has always been uncomfortable. Minor Sparks have generally been treated almost as menials -- servants who are useful but distasteful. Powerful Sparks are rivals, and treated essentially as foreign but hostile Royalty: deserving of respect, but not good people.
But the Duchy has been weakened over the past century. The Spark Wars have decimated large parts of it; worse, they killed off most of the male heirs. Indeed, the only reason the Duchy survived was due to the Heterodyne Boys providing them with new, enhanced world-class defenses. By the present day, it is ruled by a fairly elderly Duke of frail health. After having two sons killed, he has only one remaining heir: Princess Zulenna.
When Zulenna was killed by DuPree, towards the end of volume 3, word got back to the Duke quickly. The word included the fact that Klaus was trying to revive her, but the damage was done: no Revival has ever ruled a truly Noble land, and none will do so if he has his way. He had a heart attack on the spot, and barely survived. And now, there is a nasty question of the succession.
There is one other candidate for the succession: Zulenna's cousin Marie, who is the sole child of the cadet branch of the family. She ran off almost ten years ago, shortly after discovering that she was a Spark -- she knew her family's attitude towards Sparks, and that it would mean servitude or even death for her if she was found out. So she ran off, encountered the Circus, and wound up marrying Master Payne.
So Zulenna has been given the most humiliating task possible: finding her own replacement. As a Revival, she is no longer a candidate for the throne in her father's eyes. Moreover, she has begun to show Sparkish tendencies herself, and is afraid that she's about to become one. She fears further humiliation, but she is utterly committed to do her duty to her people. So she intends to find Marie, drag her back to become Duchess, and then go vanish herself.
The intended setup here is that it's really unclear who winds up with the succession, but it's mainly for Zulenna and Marie to work out together. They are really the only viable candidates, so the fate of the Duchy is in their hands. Both are strong-willed women, and if they choose to work together, they can pretty much decide how things are to work.
To make this plot a bit juicier, Zulenna and Marie should have significantly different visions for the Duchy, or at least different priorities. Zulenna is a traditionalist, who believes that the highest priority is the defense of the realm, especially from the rampaging Sparks: she genuinely believes that, much as she might hate the Wulfenbachs, they represent a necessary force of stability. Marie, OTOH, thinks that the welfare of the populace is most important, and has long been distressed at the way the excessive focus on defense has tended to detract from that. (Not "welfare" in the modern sense, but focusing more on the people and less on the castles -- a very 19th-century liberal view.) Basically, we have the conservative vs. the liberal. Each should know that there are political factions within the Duchy that supports their views, and Zulenna should be written so that she will "interview" Marie before she formally offers the job. They should be given every opportunity to disagree here.
Egging both of them on should be a "Jiminy Cricket" of their respective viewpoints. On Zulenna's pro-defense side should certainly be Von Pinn, possibly Z and (most intriguingly) Wooster trying to sew a bit of subtle dissension between her and the Wulfenbachs, by simply reminding her whenever possible that the defense of the Empire is more important than any individual Duchy. (Maybe by pointing out the casual way that DuPree killed her and Klaus brought her back?) On Marie's pro-welfare side should be Payne, who has long speculated at her what she might be able to do if she were to wind up on the throne, and maybe Zeetha, with stories of the perfections of Skifander and strong opinions about how a state is ruled well.
Ultimately, it probably all comes down to Gil -- if the two of them can't come to an accomodation, the Wulfenbachs are the ultimate authority, and Gil's recommendation is likely to carry the day. I anticipate a likely outcome of either the two of them agreeing to in some way share power, or to petition Gil to make a decision.
Should write this in some fashion that puts time-pressure on it: something that will happen soon, forcing them to make a decision. Otherwise, this plot is a bit too easy to put off until later. Probably make it clear that, at last word, the Duke has only days to live; for all they know, he might be dead already. And the vultures are circling: both of them know that there are several minor noble families that would love to take over, but neither have much respect for them -- they are mostly greedy bastards, out for their own gain, caring not a whit for the people.
Characters In Game: Marie Payne Von Pinn Wooster Z Zeetha Zulenna
All Characters: Marie Payne Von Pinn Wooster Z Zeetha Zulenna