One site can have only one domain, though you can have as many sub-domains. Well we can suggest you if you give us the previous name of your site which can relate to anyway. So please mention some more details.
If you don't have a method of marketing in mind then building a forum is very hard. People prefer to chat about things on popular social networks. You have to ask "why would anyone sign up for this?" Especially if there is tons of competition out there and the forum is not popular imo your chances of building an active community are very small.