Man, oh man, this is a pairing that lives or dies by the power imbalance, and the best fics really dig into that chasm. Dipper's this anxious, curious kid up against a literal chaos god who sees him as an amusing pet or a fascinating puzzle piece. The tension comes from whether the fic frames Bill as purely predatory or if there's some twisted form of mutual fascination. I've seen stories where Dipper's intelligence and stubbornness become a kind of leverage—Bill can't just force him into interesting conversations or games, he has to engage, which gives Dipper a sliver of agency.
That said, a lot of it gets really dark, really fast. The 'consent' issue is obviously huge, and some writers handle it by having Bill reshape reality so thoroughly that Dipper's resistance becomes part of the 'fun,' which is... uncomfortable but compelling in a horror sense. Others go the 'redeemed Bill' route, which honestly feels like it misses the point of the dynamic entirely. The appeal, for me at least, is the terrifying push-pull of a mortal mind trying to outwit a cosmic entity that doesn't even operate on the same moral plane.