Edge of Midnight spoilers and some Thoughts below:
On rewatching, Lethica states she’s thinking about staying in Druskenvald and that she feels it could be home to her (IIRC) and Marius says he intends to go home, that there’s something he has to do.
I wonder if—given they both survive the campaign and make it out still Themselves—there’s an outcome where The Dutchman Must Always Have A Captain. There must always be a lord or lady (or lord and lady) of Druskenvald. Maybe if Phillip and Adela don’t make it out, Marius and Lethica step into their positions. If Lethica wants to be able to stay, and Marius wants to leave, maybe that will manifest as walking between the worlds on the Ghostlight Express, as Phillip and Adela do.
Edited to add that my part of my reasoning is the encounter in their return to the Crooked House where Marius and Lethica had to take the positions of master and mistress of the house and symbolically lay in the bed together to bridge that symbolic gap. Nikkie does nothing without nuance and reason, so I’m wondering how the meaning and significance of that moment is going to echo through going forward in the campaign.