The problem with fanfic writing is the fact that many canons, sooner or later, simply run out. In the case of TMNT, no worthwhile new stuff besides a few sporadic comics has come out since the last movie. It's pretty much been a two-year dry spell. After that much time, writing from the same material over and over again starts to feel a little stale. Ideas run dry, and authors lose interest when all inspiration is two years old.
Ending this fandom famine is beyond the power of the fans themselves—at least, we cannot influence each other enough to revive the fandom as a whole. The event that will inspire TMNT artists and writers again will be the introduction of new, quality TMNT canon. The upcoming movie might do it. I doubt Turtles Forever, as good as it looks, will do it because it's barely even advertised, and the only people who will know to watch it will be kids and hardcore fans. It's only showing once, to boot.
I wonder what would happen if fans started sending letters and emails to Nickelodeon, expressing hope that they will take the franchise in the right direction and specifying what that right direction would be, citing examples of the first comics and the new animated series' first four seasons. I wonder how they're supposed to know the Turtles fanbase has grown up unless we tell them.
I wonder how many people would be interested in a spam project like that. I could do a little research to find contact information, write up a couple generic letters for people who have no time to write one of their own, things like that. The hardest part would be getting the word out. Still, if it's spread to enough people, it might yield some interesting results.