i think a big part of why adventure time spin-offs aren’t as grating as most spin-offs, and why people enjoyed distant lands and fionna and cake, is because adventure time as a series was never afraid to depart from the main cast for a moment and explore the world through the eyes of a background character, and the spin offs can follow that example and run with it.
3 out of 4 episodes of distant lands introduce a new supporting cast and a new leading deuteragonist (Y5, Glassboy, Cadebra)(together again is an outlier because it ties into the emotional themes of that episode). Marceline and Princess Bubblegum have a history, whats up with that? maybe Glassboy can tell us! Oh, by the way, meet Glassboy! :D
it gives weight to the very well established idea that Ooo is a populated place with a history and full of people with stories to tell. Which is neat, and the worldbuilding of Ooo is a great jumping off point for artists to tell their own stories, expanding the world even more