The Japanese version of the previously mentioned Official Smash Brothers site has posted a section where users can submit an idea or suggestion for characters and music that they’d like added to the game, as well as a general contribution letter. There is also a section to view other people’s input in which it seems that game director Masashiro Sakurai actually responds to about one in 10 letters with a “Oh, that’s a cute idea,” or “what a good song that was!”
Thanks for the tip Will!