I was doing that from a bussiness prespective, and people are on their edge with DK characters, and Sonic. I would want to see a DK character immediately, but their feeding us little by little with these trailers. Showing items, returning characters, and only a few neat new characters would be the correct option for a third trailer.
Showing cool new items, new moves on Melee characters, some more smash token attacks, and a few new pokemon from the pokeballs. How would it not be cool to have a trailer end with a sweet new 3rd party character, that noone whould expect in this trailer, or some 1st or 2nd party character who would blow the socks off players, and would be shown owning!
As for a fourth trailer...that is when you bring in Donkey Kong. Show the characters, then DK comes, beats down Mario and Wario, then you see DK go flying, & Brawl takes a huge turn in one attack. The thing that hit DK is Sonic, and then it shows him fighting Meta-Knight, and then use his Smash Token, thus becoming Super Sonic.
Then it shows Sonic get slashed, thus having Marth return, but it wasn't Marth who launched the attack, it was Ike! Ike and Marth go on fighting, when Bowser goes nuts on them! then Bowser get headbutted by Diddy Kong, and Diddy Kong gets crazy and runs over Ike and Kirby. Then, King Dedede enters by falling and hitting Diddy with his hammer, followed by Mario
Then Kirby & King Dedede fight it off. Then it the last few scenes it shows Ice Climbers, G&W & Ness fighting each other, as well as Pit and Fox. Then in the last few scenes it shows new courses, the new Final Destination, as well as 1 or 2 new Donkey Kong courses, & a Sonic course. Then at the end it shows a newcomer jumping and lands on the stage...Shadow.
During this they show off new items, such as FLUDD, Vacuum, and Shadow Paintbrush. Also more non-playable characters: goombas, koopas, kremlins, and redeads being used as throwing items, as well kremlins chasing kirby, and redeads chasing fox, whilefox trying to kill them faster with his blaster. It closes with Ness, Snake, Kin Dedede, and Pit just brutally beating each other. And ya, kinda a copy of 1st & 2nd trailer.
NOTE: I just made all this up, and there isn't a 3rd or 4th official Brawl Trailer Out. These are jsut ideas from a bussiness and somewhat a fan perspective. I can barely wait for Brawl, my mind is going mad!