Moderate or eliminate the marketplace (even if temporary)
I used to frequent the TF multiple times a day back in 05/06, and a big part of that was the amount of quality designers here. Not saying that everyone isn't talented or putting anyone down directly, but the quality of posts in the market place are definitely not what they used to be, and I bet it has a direct impact on whether people stay or not.
Opening the marketplace and finding several designs for 50 dollars that are not up to par with other places on the web can turn new visitors, or returning visitors off that there are not as many like minded individuals here. Its the same theory that applies with some video games. I used to play Wow, and when they made it so everyone was able to accomplish everything, and achievements became less special, the hardcore's and talented individuals moved on. Once they moved on the next best players took there place, but also started to find the game less fascinating because the competition was meager. This continues until the majority of anyone above casuals kept playing, and then it is impossible to bring new interest into the game.
The same principal kind of applies to facebook, although facebook is forever growing. Many of the original students like myself who got on when there school was first opened on facebook no longer use it at all or at least much less. The exclusivity is gone, our parents, and younger siblings are on there and while the idea is still great, and yes it does grow, a lot of the people who helped build it are no longer active users. In the future this could also trend into a decline, but only time will tell. The analogy of Wow has 10 million or had, and facebook has nearly 1 billion. It may take a lot longer.
Point is, if you could clean up the marketplace and help reduce the spam that ends up there, and try to reignite discussion on everything else and have designers improve, it could drive new users to read and use TF. If you moderate sales, only approving quality ones, it may attract new users. It may also make current users work harder to improve, and thus increase the quality of the user base as well.
Just my 2 cents from a long time member, even if I was never "super" active, I do still frequent TF occasionally, but often click away because of what I find/or don't find.