I'm pretty sure the leak NEVER said that the list consisted of ALL the newcomers. We still haven't seen hide nor hair of K. Rool and Ridley. And if Rosalina can escape notice, others can as well.
People think the leak is all there is because:
Sal leak + confirmed characters + Brawl vets - Snake/Squirtle/Ivysaur = 47 characters
As it happens, 48 spots fit perfectly on the character select screen, with the last one being the Random button, not to mention it's approximately the number many people were expecting. Below is an example I whipped up; forgive the ugliness.
All in all, it's certainly possible we get more characters, but right now it's all just too perfectly lined up for some people (even if their favorite characters didn't get in).
As for Lucas being gone, that's actually part of the latest leak. Right before E3, the Sal leak got another addition, which:
- reiterated Chrom, Shulk, Choir Boys (or whatever they're called), Pac-Man, Palutena, and Miis
- unlike the original Mii leak,
specifically called them "Mii Fighter"
- claimed the development team was going back and forth on dropping either Ness or Lucas
- claimed DLC is coming
Frankly, if a fourth character gets dropped with Snake and the Pokemon, Lucas makes the most sense. He's the one and only character on the whole roster whose game is still exclusive to a single region, Ness has been in every Smash game and recently had his game rereleased on Wii U, and Star Fox is waaay more important to Nintendo than Mother in both popularity and number of games so it's more likely Falco and Wolf stay than an extra Mother rep. There's also the fact that Mother is the one and only Nintendo franchise that is currently DEAD, as in the creator himself said 1-3 were it.