A huge castle where you can learn magic, explore and live adventures with your friends? There's a reason there was so much success for that saga.
JKR is a horrible person, but that universe is still there for people to create and share stories. There are a few fanfictions out there that are better than the last books.
What are your hobbies or interests? There is probably a community that could use more discussion indeed. I usually recommend moderators of niche communities to set up a weekly thread so that people can discuss things. The best is to have a few users posting daily, but that's harder to find the most niche the community is (hence why focusing on more generic communities probably makes sense with the current userbase)
Wow that's... A lot