I get a strong vibe that between Unwound/Lost Future and the prequel trilogy…there's sort of, I don’t know how to put it, two separate storylines running parallel to one another, and connected themes. Past and future. Ugh, this is going to sound pretentious and dumb. I am sorry for being pretentious and dumb, y’all.
Layton’s “arc”, if you can really call it that, is one of reconciling with his past. Let’s look at the sequence of events. Losing people he loved and not being able to know what really happened to them, his declaration that “someday I will solve that great puzzle meant only for me” in Specter, reunion with people he’d lost, and finally him looking peacefully relaxed while not wearing his hat* in the cutscene after the credits in Future. Hershel Layton’s story is a story about closure, and itself has a strong sense of closure to it. It would feel awkward and unfair to try to force him back into the front-and-center.
* Layton taking off his hat both here and in the cutscene prior to it is pretty significant, character-wise. Hershel has a tendency to deal with grief by tethering himself to the desires of the dead - devoting himself to puzzles and archeology after losing Randall, devoting himself to becoming the very picture of a gentleman after losing Claire - and his hat is perfectly symbolic of this tendency. He’s unsure of it when Claire first gives it to him, but after she dies he never takes it off, not even in situations where any other person would. Taking it off while relaxing in his office represents a move out of neurosis into a healthier relationship with his past, accepting it as part of him without letting it rule him.
Meanwhile, Luke’s story is one of a boy who feels stifled and alone in his hometown (note his lack of friends his age, how he expresses dissatisfaction with Misthallery and desire to be in the big city of London), but then gains a mentor and confidant in Hershel Layton. He leaves his parents to act as a traveling apprentice to his mentor, learns from said mentor while accomplishing feats of heroism alongside him, and at the end is forced to part from his mentor. It’s a simple coming of age story, although Luke feels he’s not ready for it to end - “I’m not a gentleman yet!” Luke’s story begs for continuation, for the player/reader of the “text” to get to watch and experience his continued growth from a a boy into a (gentle)man.
Also, Luke, by the time that he’s settled into his new residence and decides to send a letter to Layton, has his own logo based on the Professor’s. If that isn’t symbolic of him beginning to come into his own as a true successor to his mentor, I don’t know what is.
And that is why I believe that
Aang can save the world that if Level-5 decides to continue the series, it will be via a change of protagonist from Layton to Luke. Because it thematically kind of makes sense?