I think it's very useful for community growth and collaboartion to have project forums on a site that has a general open development picture. Switching between communities (and discovering new ones) is made easier this way.

There's nothing preventing us from making an entire Flare forum section here on OGA.

When Flare becomes its first game I'll probably buy a domain for it and set up a nice website.  For now this forum is serving me well -- plus, not having to maintain a separate forum is saving me a lot of time (invested in more Flare code).

I don't think good (feature-wise) forums attract more posters. My point is that expanding forums should follow the increase of popularity. Once this forum's features are not enough, add a better one. For now it's enough.

In similar vein, don't add forum subsections before popularity comes. Just because you  create a subforum doesn't mean people will appear out of thin air and fill it. Add subsections once people start complaining about having to sort out X posts from Y posts. I think it's better to have smaller number of used subforums that a bigger number of empty ones.