If you don't feel comfortable writing a pairing or type of relationship (slash, etc.), don't offer it! If you can only write AUs for a fandom, don't offer it! And if you mess those two guidelines up and get a writer you don't feel comfortable writing for, please consider defaulting instead of writing something the recip doesn't want and including an A/N about how hard it was to write because you're not comfortable with the ship uwu. I doubt any recipient is going to feel good knowing their author hated writing their fic.