The boomerang may be changed to the beetle
The way he will warp into the stage may be different *ie, he flies in on his bird*
I hope he will be able to skyward strike (my user name is based off that move... kinda)
Hopefully he will be faster and less bulky. Link is almost bottom tier in Brawl, hopefully he will be better in SSB4. (not that I care for Tiers, I'm boss with Ganondorf, TAKE THAT tier list!)
I would like to see Link remain more or less the same (because I freaking RULE with Link), but it might be neat to see some kind of new attack to replace the boomerang (as the new Link will most definitely be Skyward Sword Link, depending on the release date of the new Zelda). As people have said, the beadle may be a suitable new attack. I would also like to see a new final smash. Perhaps that would be a good place to incorporate the Skyward Strike (Like maybe every time final smash Link attacks he shoots a Skyward Strike, or something)? I would like to see another young Link, but I would like to see him much different than the other Link this time around. No two characters should have the same move-list.
His moveset will probably be about the same, but he'll be updated to be the Link in Skyward Sword, since that's what they did with Brawl and Twilight Princess.
Maybe some of the new items from Skyward Sword will make an appearance. Who knows?
Link should be able to use the beetle and have the whip instead of the clawshot. He could use a skyward strike, and he could have his loft wing help him up when he used an up B, but I think spin attack is better. I think that overall he should stay the same though, just maybe get faster.
I'd like to see (I know this isn't completely link) Wolf Link & Midna as a character. I wanted this to some extent in the previous one but now I really do want it.
As for Link from SS I see the Skyward Strike becoming a B function with B switching between items/attacks. So it would have the Bow, Skyward Strike, and the Whip. Skyward Strike charges (i.e. think samus), Bow is the same, whip can quasi-grab for either various command grab type throws.
Bside becomes the Beetle and by holding B while flying the Beetle. When you let go off the B button the Beetle drops the bomb.
Bdown is item/attack switch as I mentioned
Bup is still the spin attack
Grabs with Double Clawshot (new throws but different from whip).
I am sure as hell going to miss Twilight Princess Link and the Boomerang. I spam it like there's no tomorrow. It's in most of my combos.
The SSB4 will have different colour schemes for all the characters. Wouldn't it be cool to have costume changes (like Wario in Brawl) instead of just different colours? Maybe Twilight Princess Link could be a costume change of Skyward Sword Link. The same could also happen with Young Link/Toon Link, keeping all the same moves and only changing their appearance and voice.
One thing that would be awesome in SSB4 would be customisable move-sets, so that you can choose what each of the special moves are and assign them to your name.
What I want to know is if Link and Zelda are updated to Skyward Sword incarnations, what happens to Ganondorf and Sheik? Would they we left out all together or replaced by Ghirahim and Demise? I would like to see Ghirahim be playable, but Demise? Maybe as a stage hazard.
Also, it would be cool to be able to roll bombs with Down-B be an option as opposed to only throwing them.