Back when I first started reading fanfiction, I only read RPF (real people fiction), aka One Direction. It took me a long time to figure out why people read fanfiction about TV shows, movies, books, etc. that had set source material. I thought it would mess with my feelings about the original works too much and I was too set on keeping source fiction and fanfiction separate to really enjoy it.
Something else that I realize now that I didn't back then was that up until probably college, I didn't ship anything that wasn't canon at least at some point. There are shows or movies I've rewatched more recently and realized that there are pairings I ship in them that I never did before, which has made me want to seek out fanfiction that stars those pairings in order to fill that void that the source fiction created.
But then I found a show with a pairing that was canon that I needed more from (they broke up in the show and I cried a lot). I got into "Shameless" during the summer before my junior year of college and shipped Gallavich with every fiber of my being, so I left my 1D Larry fics behind for a little while and read as many Gallavich fics as I could.
When I get into a new fandom for fics, I tend to start out reading "canon" fics—even though by definition fanfiction can't really be fully canon, but whatever—to get used to reading this new pairing before getting into any alternate universes or crazy premises. By the time I've read all the canon that appeals to me and I'm ready for AUs, all bets are off. There are very few premises or tropes that I haven't at least tried reading, but there are a couple I will always stay far far away from (I'm looking at you watersports tag).
There are still certain fandoms I'm early enough in that I don't feel ready to move past basic AUs with yet, even if it's an AU I would read under a different fandom or pairing. For example, I very recently started reading "Harry Potter" fanfiction—I know, I'm super late to the game—and am very picky with the types of fics I will read. But as far as "Teen Wolf" or One Direction (even though I haven't read a 1D fic in a while), I will basically read anything those writers want to throw at me.
Do you guys have similar fanfiction journeys? Do your reading preferences change with each fandom, too? Send me a message or leave a comment and let me know!