It's kinda wild how much traction Finn and Marcy crossovers get. The 'What if they met as kids?' scenario dominates, obviously, because the tragic potential is off the charts. You get a ton of fics where Finn washes up on the shore of Ooo, but instead of the Candy Kingdom, it's Marceline who finds him. They both become these feral, parentless kids surviving together, which fundamentally warps the canon timeline. Marcy never gets that lonely, jaded edge, and Finn grows up with a much darker, more chaotic worldview.
Another big one is the dimension-hopping or time-slip plot. Finn stumbles into the Mushroom War era, or pre-Crown Simon's New York, and Marceline has to protect this weird, heroic kid from her own traumatic past. The emotional core is usually her seeing Finn's unwavering goodness as a painful mirror of Simon's, forcing her to confront what she lost. It's less about romance and more about a weird, familial grief. I've seen a few where Finn is the one who helps her remember her humanity, long before Bubblegum shows up, which always makes me wonder how that would change her eventual dynamic with Bonnibel.
Honestly, the best ones aren't the epics, but the quiet slice-of-life stuff. Finn teaching a cynical, thousand-year-old vampire how to have fun again, or Marcy accidentally becoming his weird monster-hunting mentor. The tone shift is everything.