or they'll just rebel against the gods and crystal and be able to overcome the starscourge anyway because reasons since the lore is whatever in the alternate reality, that could happen lol
Also, it's worth pointing out that there's nothing in the descriptions that we were given that specifically suggests rebellion against the Astrals. The description of Noct's episode says, "Parting ways with the Astrals, Noctis embarks upon his final battle in order to attain the ideal future for all his people," which doesn't make a whole lot of grammatical sense if Noct is actually fighting the Astrals. "Parting ways with" may be an ambiguous phrase, but its connotations are soft -- it would be more appropriate to re-render it as "leaving the presence of..." or "choosing a different path from..." than "rejecting..." or "rebelling against...."
Rebelling against the Crystal seems even less likely, given the Crystal's crucial role in healing Ignis in Verse 2 of his episode.
Not to mention, while neither the original game nor its Royal Edition provide any meaningful hooks for rebellion against the Astrals, they do provide meaningful hooks for Ardyn's redemption and the Crystal's capacity to offer mercy to those who willingly sacrifice themselves (both in the aforementioned Verse 2 and in the implication that Noct and Luna's existence in the original post-credits scene appears to be different somehow than Regis', even though they're all dead).
The solution I offered maintains continuity, further explains Episode Ignis' Verse 2, and allows for a meaningful Verse 1 in all of the upcoming DLCs. Going the FFXIII route instead would be the worst thing they could do. =/