My thought would be last phase of Carapace is completely replaced by N'zoth, and N'zoth would only have one upstairs/downstairs cycle, then it goes right to the end. So it would go Carapace P1 -> Carapace P2 - > N'zoth P1 (upstairs/downstairs) -> N'zoth last phase. Mythic only stuff would add at least one more phase I guess. I mean, a lot of mechanics would need a re-work but I think it could work on paper and last bosses should feel more 'epic' in scope and take longer.
Oooor there may be a reason I'm not a game designer.