There has always been fanfiction, that’s what literature is if you want to be poststructuralist about it, but with these here internets there’s ever so much more of it, so many more variations of the spectrum, and within easier reach, than ever. Most authors nowadays therefore have a policy about fanfiction, usually ranging between ”please do but don’t tell me about it” to ”I’ll sue you if you do”.
My fanfiction policy is: if you want to give my naughty pirates further adventures, pleeaase do! I’d love it if you did! And also pleeeaaase tell me about it! If you don’t want to post on your own blog/etc, Archive Of Our Own is the place for fanfiction at this point in time. You need an invite, but as far as I remember, that happens quite quickly *. Another option is to make an anonymous or otherwise submission to my Tumblr. (Also, please let me know whether you want me to share it/make it public or not.)
( * And then, you can proceed to search AO3, to see if anyone else ships that pairing from that book you read 20 years ago. The odds are probably better than you think.)