It could be the final piece to your prized collection, a person you talked to before they seemingly disappeared, a story you read that has since been deleted, etc.
In my case, I really wanted to find a website called notebookinhand.com, a forum I came across while I was a teenager in the early 2010s. It was solely dedicated to people describing their hobbies, and the community seemed very nice and welcoming, and I also like how the site was designed. It looks like it's been shut down but I can't stop thinking about it!
So, tell me what's your internet "unicorn", so to speak.