Traffic is only valuable if there is a buying force behind it. Its not something you magically "create." Advertisers pay us domainers for sending real people to their pages who will probably buy stuff.
If you create a forum on a subdomain and you fill it with valuable information that makes people want to visit your forum they will. People will not visit the root of the domain because what they are looking for isn't there, its in the forum. It doesn't matter what "trick" you try you are not going to make any decent amount of money by figuring a way to drive people to a site or page they weren't looking for. If the people are not going there because they really wanted to they will never buy anything. Advertisers don't have any use for traffic that doesn't convert, so that traffic has no value.
If you really can build a forum that is filled with good information and has lots of users, then that forum will make money by the ads you put in the forum. There isn't any need or reason to drive them to the root domain, just put the ads on the forum. Domaining is neither rocket science nor brain surgery. Don't make things complicated by trying to trick the system.