Not to mention the show DK was watching in the trailer was basically what that "Spirits" mode would be... A montage of villains fighting the protagonists of their games. This is a great video and gives me hope for Shadow to be in the game as well.
I noticed the darker themes and thought it may be hinting at a story too. There sure is a nice cinematic engine going on for them not to ultimately (heh) use it lol.
The one thing I'd like to add here on top of the Dracula and the monster Hunter dragon bosses if these are indeed 'stages' then why is there only one player on the battle feild shouldn't there be more? Another thing that I haven't heard mostly anyone say about brawl's adventure mode it wasn't only one player it was a two player coop mode along with a adventure mode on top I can't say for sure. If they do announce an adventure mode for ssbu that it will have a coop option too, though it's something to think about.