Animeverse Island

You follow a girl with a strange tattoo and an outrageous ass on your way home from class, and the next thing you know you're somewhere else entirely.

A stranger explains where "somewhere else" is: an island where anime girls from different series get dumped, and you weren't supposed to end up there. That's the whole setup. You know you're an intruder. You stay anyway.

From there it plays out as a sandbox dating sim, working through the island's residents one relationship at a time, nudging things from awkward first meetings toward whatever each girl is willing to give. No grand quest, just a trespasser making the most of a mistake nobody's bothered to correct.