I would think the biggest battle would get the forum up to speed with consistent visitors. You can monetize with contextual links/banner ads, or even set up an Ad server and promote CPA offers. OpenX is a decent open source Ad server.
Another thing is if you can get that many users, it shouldn't be hard to turn around and sell the forum for a decent price.
You could even add a premium recurring membership depending on the niche as well. I've heard a lot of successful affiliates that started with forums, so there shouldn't be an issue with monetizing it, and you'll have the freedom to test since it will be your forum!