[MaskedRiderAsakura]

So my hunch is that Aruto is the show's Final Boss and it will take all the Riders (Vulcan, Valkyrie, Jin, Horobi, Thouser and maybe Naki and Ikazuchi if the show remembers them) to take him down. Aruto, realizing how dangerous he is now, gets the other Riders to kill him.

Then a Humagear is created that is modeled after him to carry on his dream, much like what happened with Soreo Hiden. That way, kids won't be left too traumatized and Aruto can appear in the inevitable Reiwa crossover movies.

Or maybe they pull a Kamen Rider Blade on us, and Aruto just straight up distances himself from humanity and Humagears. Hell, maybe that was his plan all along, to take Ark for himself and carry that burden.