The truth is that while newcomers do sell the game, the big ones are already in. The only ones who'd whip up hype comparable to that of Wario, Diddy Kong and Dedede are Mega Man (who's in), K. Rool, Ridley and Little Mac. Palutena could be argued, but Kid Icarus still isn't so big a series that I think a secondary character from there would be anymore than Lucas hype for the general public.
The rest of the likely characters, like Mii and Pac-Man, have been quite controversial, and I'm fairly certain they'd have a mixed reception, as we've already seen with the leak.
My point is that we really don't need a lot of newcomers this time around. It's not worth it cutting characters like Ike, Wolf, Lucas or Lucario just to get characters like Krystal and Waddle Dee in instead, they aren't as well-known or respected in the Nintendo community. Heck, when it's characters like those that have a chance to be at the chopping block you know you already have a good roster. It was easy to cut characters like Dr. Mario, Pichu, Young Link and Roy because they frankly have no place on a Nintendo All-Star roster to begin with, and were only included as last minute clones because the roster was lacking in quantity. Mewtwo didn't really have much reason to be in Brawl over other characters either, considering the only thing he had going for him was his popularity in the Pokémon community, something Pikachu and Charizard already outdid him in. Then you could always argue that he should have gotten in instead of Jigglypuff, but that's another discussion.
I personally don't see any reason to include a cast of 12 newcomers, because I frankly don't even see 12 additional Nintendo All-Stars who'd all be worth enough to cut any of the Brawl characters. If we only got the Little Mac, K. Rool, Palutena and Ridley from here on and out I'd personally be very satisfied. I don't care as much about the amount of newcomers as much as I care about who they are. But knowing Sakurai, that's not what's going to happen.