Can't believe I'm getting myself into this...
Smash games tend to draw from the most recent generation of Zelda games for their inspiration. Hence Twilight Princess Link, Twilight Princess Zelda, Twilight Princess Ganondorf. Since there was no TP Sheik, and no prominent replacement for her, they used a concept model that came about during that game's development. Since there was no version of Young Link in TP and the closest replacement would be Colin (who did little besides get captured and be inspired by Link), they decided to pay attention to another section of Zelda games, the cartoony ones, for a version of Young Link. Thus came Toon Link.
Assuming the Wii U Zelda game is released after Smash 4 (which strikes me as likely), the most recent Zelda game will have been Skyward Sword (not counting the WW HD rerelease). And even if it comes out before, the two releases will probably be close together, so unless the Smash team is collaborating with the Zelda team, I think we'll probably see Skyward Sword characters, not Zelda U characters.
Thus, I think it's safe to assume we'll be seeing Skyward Sword Link and Zelda. Whether their movesets will change is a different question.
What, though, of Ganondorf? There was no Ganon in Skyward Sword. The main villain until the end was Ghirahim, and he hardly seems like a replacement for Ganondorf. There was also Demise, of course (play Skyward Sword already if that's a spoiler for you), and though he does bear many resemblances to Ganondorf, I don't know that Sakurai would see fit to replace the main villain with his predecessor in one game. So what will they do?
Wind Waker is being remade in HD on the Wii U. This provides the Smash team with updated models for Toon Link as well as a Zelda-based reason to keep him in the Smash franchise (nevermind Phantom Hourglass and Spirit Tracks). If they decide to keep a young version of Link (which Sakurai has stated is important to him, and with which I fully agree), there's no doubt in my mind that they'll use Toon Link again. And that game had Ganondorf in it! If they used the Ganondorf from Wind Waker, it would solve the dilemma of Ghirahim or Demise as opposed to Ganon.
The only Zelda vet that leaves is Sheik. As in Twilight Princess, Sheik was not in Skyward Sword. Now maybe there will be concept models of SS Sheik yet again, and since Zelda games usually deal with story last, it's perfectly possible those models exist. But if not it comes back to the SS Ganon question: fabricate a model, or use a character who looks and functions similarly to the original character? The character I refer to here is, of course, Impa. Here's the problem with that though: Impa is an entirely separate entity from Zelda. A transformation wouldn't work; the closest it would be is a tag-team swap, which would just be silly (imagine if Diddy and Donkey Kong occupied the same character slot and you could swap between them at will by tagging one out). So the debate is: do they change Zelda's moveset so that she's a standalone character, and Impa takes Sheik's slot, also as a standalone? Or do they keep Zelda the same with a model they created for the game?
There is precedent for creating models. There was no 3D model of Pit before Brawl, no 3D Ice Climbers before Melee, no 3D Ness before Smash 64, among others. And I think they'd probably make that choice before they risked alienating Zelda/Sheik players. On the other hand, there are people who play Zelda OR Sheik, who I'm sure would benefit from having a down-B move. But another way to get around this would be to use Wind Waker again, and have Zelda and Sheik become Zelda and Tetra. Tetra could easily fill Sheik's role--hell, they could even have the same Final Smash they do in Brawl--and since originally there was going to be a Toon Zelda in Brawl, I don't find this possibility unlikely.
So my proposed list of Zelda veterans is as follows:
-Skyward Sword Link
-Toon Link
-Wind Waker Ganondorf
-Options for Zelda:
---Skyward Sword Zelda (transforms into SS-esque Sheik model)
---Skyward Sword Zelda (stands alone, Impa replaces Sheik as standalone)
---Wind Waker Zelda (Tetra replaces Sheik as transformation)
Personally I like the WW Zelda/Tetra combination best out of those options, but I can see it going any of those three ways.
And what of newcomers?
Setting aside the possibility of Impa or Tetra joining the roster, since they'd likely just be replacements for Sheik, I can see at most two more Zelda characters being introduced, for a total of seven. (I know that's a lot, but I wouldn't be surprised if more Mario characters weren't introduced as well [I think it's about time Paper Mario got in], and yet more Pokémon seems inevitable.)
The two Zelda characters I think are likeliest to join depend on what happens between now and the time Smash 4 is released. If a Majora's Mask remake is announced or released before then, I would not be at all surprised to see Skull Kid join the roster. I bring this up for a couple reasons. Firstly, he won that questionably legitimate Smash Bros. character poll last year. Second, since Majora's Mask is my favorite Zelda game, I'd love to see a rep from it. And third, he'd be a pretty unique character (and another villain). However, we already have a Wii U remake of WW confirmed, so they'd probably do MM on 3DS, and that won't happen at least until they release an original Zelda game on 3DS.
If they DO release an original Zelda on 3DS before Smash 4 comes out, we may end up seeing a character from that game represented. However, as we have no info about Zelda 3DS at the moment, I can't really speculate about who that would be.
If none of those games are released or announced before Smash 4 comes out, and the Smash team wants to add another Zelda character, they'll likely either turn to Skyward Sword or Wind Waker. Skyward Sword's likely characters are Impa, who I've discussed, and Ghirahim. Personally I'd love to see Ghirahim in. He'd fill a fairly unique role--skilled swordsman with magical capabilities as well--and so wouldn't really be a clone of anyone. And he had quite a big role in Skyward Sword. On the other hand, he has a similar position as Zant (thought he is more prevalent in his game than Zant was in TP)--being an antagonist throughout but not the final boss of the game. And he's only been in one game (though the same is true of Sheik). So I can see it going either way. I think if the Smash team wants to add another Zelda slot, he's the likeliest choice, but who knows if they will?
From Wind Waker, I think the likeliest choice would be Tetra, though I think she'd probably end up being a replacement for Sheik with a Wind Waker Zelda, not a new slot altogether. If they were going to add a new slot and wanted a Wind Waker character to fill it, the only one that springs to mind is Tingle, and I don't think they'd add him, for what are hopefully obvious reasons. You could make an argument for Orca, the old man with the spear on Outset Island, but he had a minor role and was only in that game, so he'd be an odd choice.
So here's my final list of what I think is the likely set of Zelda representatives and their incarnations. (aka tl;dr)
Veterans
Link:
Skyward Sword Link
Young Link:
Toon Link
Ganondorf:
Wind Waker Ganondorf
Zelda/Sheik:
1. Skyward Sword Zelda, Skyward Sword-esque Sheik
2. Skyward Sword Zelda standalone, Skyward Sword Impa standalone
3. Wind Waker Zelda, Tetra
Newcomers
Ghirahim
Skull Kid (if MM remake is announced before Smash 4)
or
Zelda 3DS character (if Zelda 3DS comes out before Smash 4)
Thank you for reading this!