I think there's a non-zero chance we'll see Master Chief in Smash at some point. Microsoft and Nintendo have been super friendly lately, and Sakurai's been clear about wanting to make this the ultimate video game crossover. So I see it happening at some point.
Kratos would be a good fit, but Sony's been a lot more reluctant to play ball.
My hopes for Kazuma Kiryu with a Majima Anywhere ultimate move are at an all-time high since Joker's announcement.