Slash fics really pushed them together early on, back when the 'That Time I Got Reincarnated as a Slime' fandom was smaller. Now it feels like it's evolved. The newer stuff loves the dynamic of Milim's childlike innocence clashing with her actual age and power, and how Rimuru handles that. It's less about romance and more about exploring a weird, found-family bond where she's simultaneously his biggest threat and most loyal ally.
I see a lot of AUs where she's not a Demon Lord but something else, and Rimuru finds her first. Those are interesting because they remove the power imbalance from the start. The 'friends to something more' trope gets a workout there, but honestly, I prefer the canon-divergent stuff that keeps their chaotic energy intact.
Writers have a hard time making it not creepy, given how she looks and acts. The good ones navigate it by focusing on their mental connection through the 'Voice of the World' or their shared monster instincts.