I was thinking about the great titan war and the Marleyan revolution. We could see how the Tybur family secured their place in the Marley government, and how they were able to defeat the Eldian empire.
Nice idea, for me i think of the history of the Walls, especially the first hundred years, the reiss family history, or the story of pieck finger, or the Ymir Fritz"s POV about the story of her life, the next foundings, how she thinks about Eren and royalty
I was thinking of a blimp story, if we think about it the only nation that has Zeppelines is Marley, and Eldia got it when she got volunteer support, so I wouldn't be surprised if it was stolen when they returned to Marley. Following this it is quite likely that Marley has invented airships, I know the idea may sound silly, but if they do a Spin off on the Edm3d, then it would not be so rare to do a Spin off of zeppelins. It would also serve to understand Marleyan imperialism, that is, they conquer territories to get resources for their industries.