Sora and Riku were already confirmed to be in BBS, and seeing their overall importance, I'd hardly write them off from being background characters, if they want to put that detailed an image in there. (something like Goofy, Donald, Namine and Mickey were in the NA version of the Re:CoM cover)
What bugged me about it was that he put Mickey on top of everyone else :v it's not only correct plot wise, but goes against the "standard" of having the person that was shown all along (Sora for all the games, Riku for Reverse-Rebirth) be the one that ends up being shown main stage.
Not half bad other than that though o.o thanks for sharing ^_^