Yeah, i kinda had that same theory at first, and while the "game as service" theory may be better in the long run, i do hope that is more akin to the "expansion" idea...
but the thing is, if they have so much content, with probably so much surprises, why they havent showed anything yet? that scenario still lets them reveal the big guns such as some of the big name newcomers in e3, and then more info the other months up until release. but they havent shown not even a new stage, or veterans. or even inklings gameplay :s.
And also, the release date doesnt feel like a date thats have been set to prioritize smash as a full game, rather than an arbitrarty date that benefits other things besides a smash game in the traditional sense we understand them.
Still, lets hope for now that is indeed an expansion