Ganondorf
Popularity: It's Ganondorf, the first and main villain from one of Nintendo's most popular and perhaps most critically acclaimed series.
Relevancy: Did you read that last part? And there's no question that Ganondorf should get in over any other Zelda villain. He has the most appearances as the primary antagonist.
Design: He's a bit of a clone. But there's plenty to work from to give him a completely unique move set. For me the more interesting question is whether he'll still be Falcondorf or whether he'll have his actual moves from the Zelda games. He has options for sword and trident based moves, fire projectiles, energy balls (Dead Man's Volley), lightning, ground pounds, levitating, teleporting and invisibility. No matter what though, I'm sure he won't be as sucky as in Brawl. That would be depressing.
Roster and Competition: Vaati and Demise and any other Zelda characters won't be pushing him out. Sorry. Demise is really just a beefier version of Ganondorf anyway and I don't think he has much chance of getting in. I wouldn't have a problem with them stealing something from Demise for Ganondorf's moves though.
Honestly, I wouldn't mind having TWO versions of Ganondorf... Ganondorf, using moves inspired by Ocarina of Time and Twilight Princess (a sword for his weapon, Dead Man's Volley, etc.) and Pig Ganon (uses a trident, has fire bats, teleportation). Although making Pig Ganon work like Giga Bowser would be totally fine as well (although perhaps not worth the effort to make a unique move set for a final smash, unless it was a full transformation like Zero Suit Samus. Giving him a partial move set with like, 3-5 moves or so would be a good compromise).
Sakurai: Has said he's not intending on cutting anyone yet. And Ganondorf is surely not near the chopping block compared to other characters.
Ganondorf chances: 99.99%
I don't see him not coming back.
The chances of him having a significantly revamped move set (i.e. adding a projectiles and/or giving him a sword or trident) is the interesting question here, so I'm going to talk about that instead. This I can see going either way, but the evidence from Pit and Bowser suggests there's a good chance Ganondorf will finally be Ganondorf. Bowser received significant changes to his A-attacks, and it seems the main reason is just to make him better. We don't know about ALL the moves for the other veterans, but we do know their specials are pretty much the same. But we don't know to what extent Sakurai is shaking things up by changing a smash, tilt or aerial here and there for them. Pit received significant changes to his special attacks to reflect his games more (i.e. to reflect Uprising). His side-B is the Upperdash Arm, a totally new move replacing the Angel Ring (which was not based off the games), and the Guardian Orbitars, while similar to the Mirror Shield, have a totally different look and cover two sides instead of one.
Ganondorf is a prime candidate for both of those kinds of changes. Like Bowser, he needs to be buffed, and giving him access to a weapon (probably a sword, but could be a trident) is a good excuse to make some significant changes there. He also could use some additional range given his size. And like Pit, he is also a good candidate for changing his specials to reflect his games. They don't ALL need to change (Flame Choke is a good fit for him personality-wise and sort of appears in Twilight Princess) but he should have a projectile considering he uses projectiles in every game but Wind Waker.
So... I'm going to peg the chances of a significantly revamped Ganondorf (meaning, at least giving him a weapon or totally replacing a special move, rather than simply changing his weight/speed and one or two normal attacks) at 75%. It would seem strange for Pit and Bowser to have more changes than him, if you ask me. He's the veteran most deserving a revamp, rather than merely tweaking him for balance/power level.
A great solution according to me would be to add Black Shadow as a heavy/slower version of Capt. Falcon. He could be somewhere between Melee and Brawl Ganondorf with a couple new moves utilizing the horns and such, and would make Ganondorf mains very happy. That said, if Falcondorf has to die to let Ganondorf live, I'd be fine with that.
Ganondorf want: 98.5%
With a revamped move set, 100%. If the choice was Falcondorf or nothing, 95%. But I'll be disappointed if he doesn't get his own moves.
Zoroark prediction: 40%
I don't really know what people think about Zoroark, so I'm just going to guess somewhere between what others have been saying.
Nominations:
Almost there for Simon... Then I'll have to pick someone else to promote.
Simon Belmont x10