There was a loooooooot of BBC Sherlock. Maybe even more of that than of original ACD flavor. Less Elementary, but also some of that. (Tragically I did not see any Joan Watson cosplays.)
AO3 is definitely not equipped to host a community forum, and anyway there are so many in the world already. Personally, in many ways I still find LJ/Dreamwidth to be the ideal setup for fandom shenanigans. The ability to follow, post, share, reblog, easily tag, archive and search AND hold threaded discussions in the comments section is just a great combination. Plus: the ability to friends-lock your posts so that only the people you want to see them can see them.