I think that when the subspace comes to Smashville KK Slider is going to be the one to step up. He is the hero of the town, as said by Sakurai, and he's there to protect it and save the day!
However I believe that the subspace is going to be seperated into chapters because that would make it easier to allow for all the characters to fit into one big story. I'm talking the first chapter being all the people we saw in the videos and Yoshi and Link. Then another chapter chronicalling all the villains, Bowser, Wario, Meta-Knight, and the others. Then maybe another good guy story with the Pokemon Trainer, the ac character, Ike, Lucas, the Ice Climbers, and more. This way all the characters could fit into subspace and allow for a longer 1 player experience