After seeing the videos, Nintendo is treating Snake as more than a guest character. He basically has his near complete universe in the game, complete with original voice overs, dialogue, stage and a trailer showcasing his involvement in the series. This is 100 times more than he got in World Fighters Dream Mix TV!
Will other third party characters get this type of treatment? If the Metal Gear in SSB trailer was awesome, imagine how insane a Sonic and Mario rivalry trailer would be.
In fact, here's how I see it:
Mario is running around in one of the stages. Then a blue blur runs through him and stops, revealing to be Sonic. Then the two size each other other (complete with cool camera angles) and start the Brawl. Then the trailer shows off Sonic and his moves and ends with "Only one shall emerge victorious" (or something cool like that).
With the MGS trailer and videos they have the fanbase hooked. They will catch MILLIONS if they make a big "Mario vs. Sonic" promo.