Wolf is practically a given since Star Fox Zero is due Holiday 2015 and he can easily be ported from Brawl. It's not a coincidence the Falco amiibo is the last non-DLC character to be released.
In terms of the ballot, I personally doubt Nintendo is going to add a 3rd party character. All the 3rd party characters included thus far are extremely iconic (so iconic that even your nongamer friends can probably name them, or at least recognize them). Shantae and Shovel Knight are nowhere near their popularity and legacy, despite the vocal minority you often see on forums. Banjo Kazooie is Microsoft. Final Fantasy is primarily a non-Nintendo series now, and unlike Ryu, constantly changes its universes and characters (so unless you want Warrior of Light, Cecil, or Terra, FF is probably out of the question). Sora, also primarily non-Nintendo, gets tricky because he has strong associations with Disney and therefore doesn't really work well aesthetically as a standalone character. If Snake gets in, I doubt he'd be considered the ballot character just because a returning veteran would be underwhelming for everyone who didn't vote for him (and would kind of defeat the purpose of the ballot).
In terms of 1st/2nd party inclusions, I actually don't know, because I feel like everyone that should be in Smash 4 (aside from Wolf) is already in. Off the top of my head, the biggest contenders are King K Rool, Isaac from Golden Sun, and Splatoon inklings. King K Rool hasn't been that relevant since DK64, and Golden Sun has a lot of reasons going against it (only popular in West, Dark Dawn was poorly received, another anime swordfighter, etc).
So my prediction? Splatoon inkling.