I've been watching a long play of YU-NO and...man. This game probably had more influence on PSO2's worldbuilding than the actual Phantasy Star games.
Spoilers for YU-NO
YU-NO's second half transports the main character to an alternate world; while there, he encounters different versions of characters met in the first half. On top of that, one of the major plot reveals is that the alternate world is in danger of colliding with Earth; the need to change its orbit to save both worlds is the central problem of the story.The game's true ending has Yu-no sacrificing herself to save Earth (the alternate realm is kinda doomed), ultimately ending with herself and the protagonist alone in a void--it's heavily implied they return to the beginning of time. It's hardly a beat-for-beat mirror of PSO2s finale, but you can see where the idea of dragging an unsolvable problem to the beginning of time came from.
I was planning on reposting my YU-NO/PSO2 connections thread over here at some point, but i feel like i have to expand the whole thing now. I didn't even mention how the alternate world is actually a spaceship built to save an ancient, technologically advanced race from a future apocalypse they predicted, which is why there's so much time and dimesional fuckery in the first place... 🫠
anyway it's been cool seeing all these parallels. it's definitely made EP5 seem less...insane lol.