Sounds like a great idea to me, but more of a fan site type thing then a program's forum. Here, there are many of places to find different things made for the program. The list you gave would be a must for sig site, or art site in general. But this isn't an art site as much as it is a paint.net site. C4D renders are made in Cinema 4D, nothing related to PdN.
I like the idea, but as David said, this site isn't ready for it, yet.
I'd love a section to put my SigPack1. A download forum here would be a very good addition. Just paste the filefront, or other upload site's link and voila..