If you check the SSBB website, http://www.smashbros.com in the English section, you'll find it updated quite recently. It features a little of explanation of SB rules, some info on the "Battlefield" level, which I must say features a cool feature: since the level is very simple, you'll see it change from dusk 'til dawn literally.

But the "Musicians" new entry is what intrigued me the most. It says that a lot of famous musicians are lending their work to the game as new arrangements, not only Nintendo composers but other famous composers as well. I think this is a hint... and it started by saying the game features composers from games such as Metal Gear Solid, so we may see characters from other series such as Pikmin, Romancing Saga, Wild Arms, Golden Sun, Kingdom Hearts, NiGHTS, Magical Starsign and even Streets of Rage!!

But well, this is just speculation. Check the link to check it out.