Retro, 2D assist trophies? If nothing else, this pretty much confirms that there will be a billion or so assist trophies. At this point, really, what's gonna stop them from having an assist trophy of virtually every Nintendo character ever? There'll be plenty of variety, that's for sure.
Also, when this update showed up in my RSS feed, only labeled "Excitebike," I could assume it wasn't a character, so my first guess was that it'd be a stage. That'd be pretty interesting, eh? An Excite Bike stage?