It doesn't need to be their young selves. Finn is going to be of age soon after all.
I just like the idea of a sort of corruption of what was established early on. He's this sort of pure good Knight that has sworn to protect the just, and kind princess.
That wasn't interesting, but it got interesting when Finn slowly learns that pb isn't as good as he thinks. She's sorta fucked up honestly. After all it was her her clone successor that went evil with power, and Finns was the one who was naturally righteous.
But, Finn still accepts her. He's still her Knight. Even now that he's sort of fucked up with too, he still is there for her.
I felt like that was really engaging, but guess not...