Haibane Renmei, Made in Abyss Recommendations
Though they look quite different, both shows have a very similar vibe. They both take place in mysterious, unique worlds where the rules are unclear and only slowly revealed. Both shows feature positive lead characters in a setting that turns out to be quite dark in nature. In the process of discovering the secrets of the world, the main characters are subjected to severe suffering - expect some serious feels in both shows. Both have strong set of side characters, though in my opinion the main draw for both are the incredibly detailed and creatively set up worlds.
The music also is quite good in both.