This is the thread to suggest ideas for the forum, new features, discuss forum politics and all the like. If we decide that a particular suggestion warrants a discussion thread of its own, we'll make one.
This is the SMF mod site, if you're interested in possible features that we could implement. Just make sure that before you suggest a mod, it's compatible with SMF version 2.0.
Skins and themes are here. Again, make sure that they're compatible.
As for everything else, the sky is the limit. It's strongly encouraged to share if you feel you have an idea of what's good for the community. Even if you're unsure, it doesn't hurt one bit to start a conversation.