If you choose to start a membership site about say .dogs. Just dogs in general wont attract many members. You will need to choose a specific breed .and possibly even narrow it down further. Instead of Poodles, your theme should be; Tiny Toy Poodles, Miniature Poodles, or Standard Poodles, for example. Millions of people are dog lovers and the theme of a specific breed of dog should do well.
If you are really into video or computer games, start a membership site devoted to one specific game not just video games or computer games in general. People all over the world and from every walk of life get really involved with a specific video or computer game and love to talk to others about the game and give each other hints.
There are thousands maybe millions of people who love certain crafts and hobbies. However, those who love to quilt arent interested in wood working so your membership site should be about a specific craft or hobby.
Professionals like to talk to other professionals in the same field. Each professional field speaks their own language and find the same things to be interesting, informative or funny. So if you start a membership site for professionals, you should get a good response.
A membership website with the theme, Resell Rights, should do well, also. This kind of site will need to be kept updated on a regular basis.