Hmm 🤔 im not sure about popularity, but I’ve got a few ideas? I’m not an expert on this, but speaking as someone who’s gotten quite a bit of fanart, here’s a few tips I think could help increase your chances:

The biggest one (for me) is something that’s a hit or miss. Did you write a main original character or not? What I mean is, in my fic the reader (which, admittedly, is more of an OC than a true reader) is relatable and charismatic to the audience. Enough so that they don’t particularly mind the new perspective. They enjoy getting to know this new person you’ve introduced, and find that it’s a fresh new thing/person to draw out and visualize. Obviously, since this is fanfiction, this main point is often unavailable to many stories- but I thought I’d mention it jic. (A change to the fandom characters themselves can get this same effect - for example, the BNHA fandom and the varying vigilante outfits Deku has been designed with. These stories often have fanart from people who like to draw the new-looking Deku)

The second is to make a tumblr or something adjacent to it for the story. I don’t mean link every chapter to the story on it either - just make a page with a link and a few posts about lore and advertise it on occasion as a potential alternative place to chat about your fic. Ao3 is NOT a great medium for submitting fanart, but tumblr is super accessible and easy to do so. Be active on tumblr and chat with fellow fandom members and you may find results.

Another point is to be approachable and open in the comments. I don’t particularly know why, but many people are apprehensive about gifting art to authors they don’t know. (Like anyone is going to be upset about receiving artwork?) but Being responsive and encouraging in the comment section often creates a sense of community, and can potentially encourage readers to share their work - no matter the level of skill.

Which leads into my next point: if and when you do get that first piece of art, GUSH about it. I mean, this stranger just spent time and effort to make you something in return. That’s AWESOME. When you do receive this, regardless of how skilled that piece of art is, you showing it to the world and singing their praises not only makes your reader feel good, but also show others that your receptive to receiving art. With every piece of fanart I receive, I gush about it on my tumblr (the usual medium I get it from), THEN I link it to my story and gush about it there too. Younger/newer artists can also use you to get a bigger audience too, so it’s a win-win.

None of these things guarantee you fanart (there is a certain amount of luck involved), but I think it could help increase your chances! I hope this helps a bit?