I would say dont be too specific for example:
A music forum covers a massive spectrum, an RnB forum will probably attract a slightly broader range of members because it hooks into many different genres especially with all the collaborations etc.
For instance poetry is something i've seen as a subforum on a few music websites, this makes sense because songs are basically poems with music.
A forum for left handed bass players called Andy, is in all probability going to fail.
Basically, what i'm trying to say is dont be too overly specific with the 'niche' your working on and look for ways in which your forum can maybe pull in or touch on another 'niche'.