Gnome desktop is undergoing development for the next major release – Gnome 3.6. The features have been freezed and test betas are already out. This means you cannot suggest features for this release. However, you can suggest new features for Gnome 3.8, which will release at sometime next year.
The proposal was announced in Gnome devel mailing list and all developers can take part in it. Some of the features that have already been proposed are fixing/dropping of fallback mode and automatic extension updates and can be viewed at this page.