I'd say 75% of the time when I'm reading fic for a canon I'm not familiar with, it's because an author I liked was a gateway. If I know and like the author's favorite tropes, I think it becomes fairly easy to guess the background shapes of what's going on even if I don't know the plots and canon-details.
The other 25% of the time it's because it's a fusion crossover with something I am familiar with and like so rather than being, I dunno, Hannibal/Fruits Basket fusion, I'm reading some kind of neat Hannibal AU where Will Graham turns into a dog when people touch him. Or something. But since I click on these for the "high concept" I'm less likely to finish them if they don't work for me, too.
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The other 25% of the time it's because it's a fusion crossover with something I am familiar with and like so rather than being, I dunno, Hannibal/Fruits Basket fusion, I'm reading some kind of neat Hannibal AU where Will Graham turns into a dog when people touch him. Or something. But since I click on these for the "high concept" I'm less likely to finish them if they don't work for me, too.