Most characters in Brawl stand a decent chance, we just haven't developed our meta games enough, especially for edge games. That is why Snakes are almost as dominate as the space animals in Melee. Brawl's edge games will be much diffrent from Melee's, and require more precise timings and whatnot, and are generally more advanced (except less characters can spike.

). Once we develop more advanced edge games, I see Meta Knight, Snake, Pit, and others slip on the tiers. Thus-Brawl will become more balanced.
We definitely don't have any smash 64 Links or Samus', or any Melee Pichus, Kirbys, Zeldas, or Mewtwos. Still...some characters already (Booser anyone?) don't stand a chance in most, if not all situations. In fact, Bowser is probably actually worse then he was in Melee, thanks to his attacks not being overwhelmining damaging, he doesn't have huge knockback. and his attacks are either slow or hardly above average attack power. Bowser just lacks killing power, strong moves (kinda like how Ike has), and overwhelming range. And he's a big, slow, and fat target to top it off. See what I mean=??? (sorry Gimpy

)
Overall, heavy characters are pretty much going to have a few high priorety moves, and just use those, thanks to no L-cancelling. Like seriously...why the f*** is L-cancelling not even a automatic=??? It's easy as crap to do, anyways. Anyways, Ike, Bowser (even if he's basically screwed, minus Gimpy and a few other good dedicated followers), G'dizzle, and the King have to focus on move execution, and really time it right, thanks to no L-cancelling. Ike has his range, and has some fairly fast attacks in his "Leaf Combo" (thanks to that post by leaf, that is now it's official name in the holy book of Johnknight1ism!

Which I just made up 5 seconds ago!

), Ike can mix it up. G'dizzle IDK about, but at least his fB can tech chase, and his dB and fB cover a good range of area (though they are predictable). Overall, the heaviest heavy characters are basically screwed.
Brawl isn't amazingly balanced or anything. Any deaf person could see Bowser is nowhere near as good as Snake, Marth, or Toon Link. Link has apparent flaws that should have been fixed, and Samus does as well, amongst other characters as well. We've just dug into the meta game of Brawl. Japan and free loaders (nothing personal against them) have had the game for not even 5 months. America has had Brawl for about 2 1/2 months. We're finding more new techniques each week. If you doubt me, search recent threads (to futher prove my point). Snake and Meta Knight are going to go down in tiers, and will stop being as dominate, once we figure out the edge meta game for each character, and learn to counter them (as well as work the annoying spike system for some characters!

). Once we learn how to work that, Snake's recovery can easily be countered, and he won't be such a recovery God anymore.
Basically...what I'm saying is as of right now, Brawl is getting generally more and more balanced. We're finding techniques for the mid tiers-the heart of the game (basically) that allows them to at least keep up somewhat with the top tiers. Maybe they are still lagging behind, but they are still fairing fairly well against them. There hasn't been any big set back since that ever-famous up smash sliding manuever (with Snake >_<), the Ice Climber's infinite grab, and the Ness-Lucas hitstun issues (and the fact that Yoshi and Marth can basically infinitely release grab them). However, there could be a set back at anytime, with new techniques being found that could more or less break this balance.
All in all, Brawl seems to be set to have more percentage-wise useable characters and characters that can compete. However, anything can break that balance, like in the past. In-game depth can hurt or help this balance, and anything can basically happen. So far Brawl seems to be the most balanced of the three (followed by smash 64 percetange-wise, and Melee character-wise), but it can more or less be broken as of now. It's not too close to call, but the balance can easily be broken, basically. It's too early to be confident in which way it will go, but the balance seems to be leaning towards Brawl as of now. But as with everything else-anything can happen. Let us hope the balance of Brawl continues to get better and better, as it generally seems to be set to be.