Zoku Owarimonogatari

Movie finales for series in which our main protagonist enters/gets put in another world which can be described as an alternate version of what they are usually familiar with in an unexpected manner.  

From a conceptual level, Kyousou Giga and Zoku Owarimonogatari take place in a "mirror world." If you are interested in stories focused around the peculiarities of a mirror dimension and its relationship to the protagonist, either of these anime may be worth checking out (although if you've watched Kyousou Giga first and are looking into Zoku Owarimonogatari, it would probably behoove you to watch the shows within the Monogatari series preceding Zoku in order to have context to the characters and plot.) Additionally, alongside having a healthy dose of tongue-in-cheek humor and characters and settings with supernatural traits, the themes of both are uplifting, so neither are particularly dark. read more 

Both are Epilogues of their respective series that dive deep into the character's psyche and tie in the loose ends of everything that happened so far. Both series are told achronologically and this final installment really shines in the psychology of the characters mind. Both involve interesting and deeply philosophical dialogue with unique visuals. 

In both of these movies, the protagonist goes to a reversed, mirror world, through a surface in a bathroom. The original work that Zoku Owarimonogatari adapts even acknowledges this simmilarity.