Steam Workshop would be functional, it would reintegrate add-on downloading into the game like RTB did.
Another website for add-ons would serve no such purpose.
Other websites would be functional aswell, I mean we have GameFront, FileFront, ModDB, Desura, NexusMods, and Steam Workshop which are great places for sharing User-Created game content.
It would be GREAT to have more options. What is so wrong with that?