Just because DP has so many categories doesn't mean there is nothing left for you or any other site for that matter, there are still areas of SEO and the Webmaster niche you could go into. Have you thought about offering free advertising on your site to new members? This has been a good way for us to gain registrations sometimes.
One suggestion to get your forum some new members is to find some blog owners in your forum niche who has quite a big mailing list and make some sort of deal and ask them to mail it out to their list asking people to join your forum. Once you already have some users then some sort of posting competition can work very well. We recently stared contest on our forum and it is still early days but it seems to attracts some interest already, probably the main prize, 10 million is helping to get that interest.
I am here because of this forum has everything to discuss around. Well I am also working hard to make my forum best too. Although I can't be succeeded as you are. May I create a thread to promote my forum?
Come and advertise it to my forum for free you can even get banner for just few posts:advertise4you.info
i am here becuase this forum is just so busy that i need to be part of the trade in sell/ buy section and discuss things. now is your forum busy? if not then do whatever you can to make it look like it is to gain members. i started tweeting helpful threads of my forum and then started to SEO for awhile, post some articles and discuss it in some forums i've found. then joined in postloop