Ok... I'll participate again.
First off... I'm going to be Debbie Downer again, and say that ratings need to be lower, across the board except for the characters for which there is strong evidence (i.e. Roy and Ryu).
There are perhaps 2-5 open slots from what I can see (2 in the case that Lucas, Roy and Ryu account for three of them) so I will again be Debbie Downer, rating almost every character lower than the average rater. 5 slots for characters that aren't Lucas/Roy/Ryu seems optimistic to me, so I'm rating under the assumption that 3 beyond them is maybe the average outcome. That's 300% to dole out to characters. Each percent you dole out should reduce the chances for everyone else accordingly because space is limited and results are NOT independent. In certain cases, I'd say there's an even stronger relation - Dixie and K Rool's chances are very strongly linked, since the chance of both of them appearing is far, far, far smaller (<1%) than either's chance of appearing alone. Other characters may be competing in the same space for other reasons as well (such as both being sword users, etc.).
Impa
Impa has a few problems. She's inherently similar to Sheik - she's an Sheikah warrior, Sheik is Zelda disguised as a Sheikah warrior. They can make her different, of course, but she's not that different in build (unless they use fat Impa...). A fruitful direction for her could be to be the Ganondorf to Sheik's Captain Falcon - OoT Impa is a bit more solid build than Sheik, for example. SS Impa is less so - being taller and even thinner. But making her the slower, more powerful Sheikah warrior is an option along with little differences here and there. I feel like if Impa was in, they'd go for something like the Hyrule Warriors design - an updated OoT design, maybe with some TP and SS influence. Even if it wasn't SS Impa, an SS-inspired palette would be extremely likely.
Obviously Lucas is coming back, Wolf has a very good chance, and Roy might already be in development. So being similar (Wolf, Lucas) to very clone-ish (Roy) is not an insurmountable hurdle. But for newcomers, I feel like they'd prefer more unique characters. Zelda is also not sorely lacking representation, although it could easily justify having a 6th rep. Impa would also broaden the representation for Zelda out of the three Triforce holders. On the other hand, Beast Ganon would lend a pleasing symmetry to the Zelda cast (two versions of Link, two versions of Zelda, two versions of Ganondorf)...
Biggest problem? I don't think she's getting enough votes. If they take votes and don't go with some of the most popular characters (and yes, Impa does alright, but she's not in the top tier by any means) and pass over them for non-veteran semi-clones? I mean, it's not out of the question, but it would feel like they looked at the polls... and picked the laziest semi-popular option. If they're out of the top tier, I feel that more unique characters or characters from under/unrepresented franchises have the edge.
On the whole, I'd say Impa has a decent chance, possibly even the best chance of any Zelda character. Her competition is the less popular/relevant Vaati, the very clone-y Toon Zelda (perhaps less popular, but she was popular in Japan if I recall), third version of Zelda Tetra, less popular Beast Ganon (little optimism for this given Ganondorf's overall treatment in Smash) and Tingle who was driven out of the franchise by American hate. Groose might have some support, but I doubt he would be picked since he's not as important within the story. Ghirahim would be ranked higher than her, I think, if he weren't already an AT. I don't really know if ATs have any chance for Smash 4 (Smash 5 on the other hand...).
But unfortunately, a decent chance doesn't translate to much in my reckoning. Decent compared to most characters, which I would rate below 1%.
Impa chances: 3%
Chance has to have a strong correlation with votes, and the evidence I've seen is that Impa is not top 10, or even top 20. Zelda reps don't seem to be at the top of the list.
Impa want: 60%
She'd be cool enough, but she's not my top choice by any means.
Shantae prediction: 7%
She's got ok popularity, but 3rd parties need to be pretty iconic...