I think that in Reboot and Preboot the AT timeline is going to go back to the season 5 finale, where Finn loses his arm but this time with an alternative ending (Martin making the correct choice instead of running away from his son). I think that Finn's woes about the Finn Sword are going to make him visit Prismo (who's got a lot to do with the Finn Sword) and wish that his father weren't such a jerk. An evidence of his inner wish of getting his father's shit together and that, even though he's an asshole, he still admires him, would be the song that Finn sings at the end of The Music Hole. Is then the AT crew brave enough to delete the progress achieved in two seasons? I don't know.