Around September, Sakurai(creator of Brawl) had said that he wanted about 3 3rd Party characters. Currently, there are 2, Snake and Sonic.
Then, around December, a rumor around the internet had said that the reason for the pushed back release date was because Sakurai wanted more than 3.
The original release date was Dec. 3, but was pushed back to Feb. 10, which is 2 months later. That is only enough time to make about 1 or 2 new characters, so if you do the math, in all there will be about 4 or 5 3rd Party characters. With 2 down already, who do you think the others will be?
Just my opinion, but I seriously doubt it is Naruto or Leon Kennedy.