We've already gone over who will probably be coming back in Brawl, so now is the time to look at how those returnees might look like in the game. Both Link and Fox got fairly major visual upgrades based almost entirely on their more recent game appearances. How this might affect their fighting styles has yet to be determined, but I do expect some new stuff to show up (aside from each one getting a super move, that is).
Nothing particularly major for him, as I feel that Smash Bros. has always done a good job representing him and there really haven't been many DK games that stray particularly far from what we expect in recent years. Something similar to the more detailed Jungle Beat model will certainly be his visual.
Peach has been up to a bit more in recent years. Super Princess Peach, her first solo outing, gives us a few possibilities for new moves in Brawl. First and foremost, she could have the goofy talking parasol at all times, possibly using it in her A-attacks. Far more interesting (or bizarre) are the 'emotion' effects she used in the game, which included joy (health recovery), sadness (gushing streams bursting from her eye sockets), and anger (EXPLOSION), some fancy new things that could be incorporated into her moveset.
Gameplay-wise, it's hard to predict changes to a character's whose techniques were made from scratch in the first place. Since it's a minor meme now, they could try to incorporate the super ultimate annihilating Falcon Punch, possibly as a super move. But, in general, any changes to CF are going to be aesthetic, likely based on F-Zero GX, although you really need to pay attention to notice any real difference.
In Melee, Zelda was interesting because she could turn into Sheik. But what happens when she can't turn into Sheik? That's a question many people have been asking since they first saw that Link's character model is right out of Twilight Princess.
It seems logical that if Link is based on Twilight Princess, Zelda would be too. And guess what? No Sheik, or anything similar to Sheik, in that game. That would seem to take the ninja alter-ego out of the equation entirely.
I mean, they could throw in Sheik back in just for the sheer hell of it. But just in case they don't, some modifications will have to take place. Thankfully, TP makes those a bit easier.
For one, Zelda carried a sword throughout the game. She only used it when Ganondorf possessed her, but what's to stop them from incorporating that into her A-moves alongside the original magic attacks? Second, for the past two games, Zelda has been the one launching light arrows at Ganon's ass. We already have two characters with archery-related moves, but that doesn't mean it's forbidden fruit. Diversify them, it's not that hard.
Many people want to see Luigi changed to the point where he shares nothing with Mario, thus taking away his 'partial clone' status. I don't agree with this, entirely. I think Mario and Luigi should share some moves. They are the Mario Brothers, after all. The changes they made to him in Melee were going in the right direction, and so I guess some more in Brawl will be fine.
Of course, the big thing for the green guy is Luigi's Mansion, which came out around the same time as Melee (they even had the secret trophy), but was probably too close to have anything from it incorporated into the game. Now that it's been 6 years, it's all fine and dandy. Aside from a possible super move related to the game, they could possibly swap out the Luigi Cyclone (which isn't particularly essential to the Mario Bros. style) for a vacuum-related attack. As long as he keeps his fireballs, fiery jump punch, and Luigi Missile, I'll be happy.
Ah, Ganondorf. Back when the director said that characters would be "remade", he was probably the first on everyone's mind. He was originally supposed to use the giant sword from the Space World 2000 video in Melee, but time constraints ended up turning Ganondorf's completed model into a 'bodyswap' for Captain Falcon. But now is the time to fix that.
Going along the same idea as Link and Zelda, Ganondorf will likely boast his Twilight Princess design. This replaces the large sword he had before with a smaller, glowing one. No biggie, a sharp weapon is still a sharp weapon (although I would have loved to see Ganondorf wield two swords like in Windwaker).
For special moves, you could pull stuff from all of G'dorf's appearances, and maybe even a few things from the monster Ganon as well. At the front, we have his fireball throwing/deflecting from Ocarina (possessed Zelda used it in Twilight). Also from Ocarina is a jumping smash/small earthquake which could be used as a 'replacement' for Melee's Warlock Punch. In the horseback battle in TP, Ganondorf was able to make phantom warriors come out and attack you, which could be very interestingly interpreted in Smash Bros.. The rest can consist of anything they imagine that makes sense. I just hope he remains a heavy beatstick, because that's why I like him so.
Of course, if Ganondorf's super move doesn't involve turning into Ganon and wrecking up the place, I'll be surprised.
And those who stand still:
Not all characters really have much to base changes on. Characters like Ness, Bowser, Marth, Jigglypuff, and Mewtwo just really haven't had anything to base any radical changes on. With a little imagination, some new attacks could be made. But overall, there is little to no source material for visual or mechanical improvements that could be implemented.