I think by it's very nature, any forum of this kind is going to struggle to attract and keep new regular members. Without a real 'hook', you're going to struggle to stand out over all the other myriad forums doing similar things.
In a way, that's one of the real shames that Mishimi's ego suffocated DBD, because Dragon Ball was a pretty decent hook, all things considered. I'm not sure how typical I am, but I'm a regular member on only one other forum, and that's car related. That was my hook, that's what keeps me interested. Even somewhere like SLUT I rarely find cause to visit, much less post regularly because it's geared very much toward Sly's PS3 Clan, and most of the topics that aren't are broadly similar to what's on here. Not to mention all the best members there are here already anyway.
I digress slightly. To summarise; honestly, I don't think you'll ever attract huge amounts of new members, not unless you choose to specialise more in one particular field, whatever that may be.