I'd have to agree. "Max" and "no final boss" are really the only viable options; they could throw something random at us, but that wouldn't be narratively satisfying, and I doubt they'll go that route. I also doubt they'd pull a "Victor is alive and he's the final boss" twist, because there's nothing foreshadowing that and it goes against the entire story up to this point. There might also be a fight against the rival beforehand, but that'd have to be optional, especially if you befriend them - and it probably wouldn't be a very difficult fight anyways, since their threat has been diminishing with every subsequent main story planet.