That’s not such a great idea. It’d be like baking a cake (forum) and having each ingredient (topic) in a different bowl (category) and each bowl being in a different room (subdomain). Why not have all the ingredients in the kitchen - in separate bowls (categories) but all on the same table?
You can create categories and sub-categories for your topics. No sense in stirring your “cake mix” with a tooth pick. 
The old “standard” TLDs (.com, .net, .org) are cheap. There are many newer (still short) TLDs available that are cheap. The .community domains are usually 2 to 2-1/2 times the cost of those. Browse the list on any registrar. I’m sure you’ll find a good one. Once you’ve found one that would go nicely with your domain name, you can always create a subdomain “community”, “forum” or “discourse” 
and make that the home for your community. Good luck!