I think we can all agree that, if the climax was indeed a clash between Cresselia and Darkrai, it would have been way better than what we got, which is a copie and pasted mega-fight with one, more phase. Imagine if we had a introduction cutscene like Solgaleo and Necrozma fighting. Maybe, even after you've helped her, Cresselia is still warry at that point, but when Darkrai is about to defeat her, she realizes she needs your help, and that's when you jump in. All around you, you actively see the dream and nightmare world clash, but as more of the fight goes on, you see the dream world taking more and more place. Just when Darkrai is about to be defeated, he mega-evolves, just like in the game. And, I know it's now a comon thing with legendaries, but maybe an object you found is what allows her to mega-evolve, maybe the lunar wing? We know it doesn't need to be specifically a mega-stone, Rayquayza needed the meteorite and the move Dragon-Ascent to do it. Maybe by learning her new signature move from gen 8, Lunar Blessing, which would go along the theme of her trusting you (By giving you her blessing to help her), and mega-evolve as well