Impa
Popularity: Not very high. I don't see there being a lot of demand for her in Japan or the West. For example, she only gets about 10% support in Shortie's poll, coming in somewhere around 100th place in the poll. I'd be very surprised if, relevancy or not, Sheik didn't rate massively higher. This is her big problem. Outside of devoted Zelda fans, I think a lot of people would say "Who?" were she included. And if not "Who?" they'd probably say "Why?"
Relevancy: Impa is one of the most recurring characters in Zelda games (and instruction manuals), and while she has often appeared as an older or fatter woman, in two of her most notable appearances, Ocarina of Time and Skyward Sword she was portrayed as a bodyguard, with magical abilities and a competent warrior. She would easily fit in. While she's not as important as some like Vaati or Ghirahim, she's been in many more games than other suggested characters. Of the "contenders" only Tingle has more game appearances (when you count his spin-off games).
Design: As a Sheikah, her likely move set would be very similar to Sheik's. So it does seem that she'd either be a semi-clone of Sheik or Sheik would have to go. But if it's the Skyward Sword version, she has a graceful build and would fit Sheik's move set perfectly. Add a new down-B and some tweaks to Sheik's moves (the whip kinda sucks, and there maybe should be some more magical look at least) and she'd be pretty cool.
Roster and Competition: But Sheik is the big obstacle. I think you can plausibly argue that Impa is more relevant than Sheik, and not just due to recency. Sheik's one big appearance was in Ocarina of Time, and that was quite a while ago. Impa, on the other hand, had one of her biggest roles yet in Skyward Sword, where she was also quite the bad-ass.
Nevertheless, Sheik has appeared twice, and her lack of relevance wasn't a hindrance in Brawl - they just gave her a Twilight Princess art style even though she didn't even appear in it. She's more well-known and from the most critically acclaimed video game of all time (reports of Mario Galaxy beating OoT should really just say it's a statistical tie). Removing Sheik would create more work, as they'd need to create new down-B special moves for Zelda and Impa, and unless Impa was otherwise just a reskin of Sheik, she'd be a lot more work than Sheik (knowing Sakurai, he'd want to give her her own flair in animations and tweak her moves more than Sheik's would've been tweaked).
And this is without even talking about the likelihood of Tingle, Vaati, Ghirahim or other possible Zelda reps.
Sakurai factors: We know that Sakurai had at least at some point thought about putting Toon Zelda/Toon Sheik (Tetra?) into Brawl. That pretty strongly suggests that some variation on Toon Zelda/Toon Sheik/Tetra is the most likely Zelda newcomer (and if the name Toon Sheik wasn't just a joke in the code, I would hope that upon reflection Sakurai would realize it should be Tetra, not Toon Sheik). Since the development period of Brawl, Phantom Hourglass, Spirit Tracks, Skyward Sword and Ocarina of Time 3D all came out, and Sakurai likely knew about Wind Waker HD early enough to take it into account. Those factors help both Toon Zelda (two new appearances and a remake) and Impa (one new appearance and a remake), but help Impa more relative to her previous importance (Skyward Sword being her biggest, coolest appearance yet). But probably no enough to overtake Toon Zelda as the most likely Zelda newcomer.
Impa chances: 3%
Inertia and the low demand for Impa make me think she's not very likely at all. The Forbidden Seven shows that Sakurai had decided on Toon Zelda out of the Zelda options before. And I'm really not convinced that Sakurai thinks Sheik is a problem that needs fixing, which is the main reason to add Impa. I don't think her appearance in Skyward Sword is enough to get her in if Sakurai doesn't have a problem with Sheik. And with Sheik still around, I don't see much room for Impa.
Impa want: 90%
Impa in Skyward Sword was really cool, as I said before. And she'd add another strong female character (while Sheik is that too, she and Zelda are still only one person).
I really like Sheik's move set (Sheik was my main in Melee). To be honest, I don't care that much about Impa vs. Sheik, as long as we get at least one of them. I somewhat prefer the Impa choice since it would mean new down-B specials for Zelda and Impa. But they could also do that by changing the way Zelda/Sheik's transformation works. But I wouldn't really miss the ability to switch between them since I rarely used it. Transformation characters are also harder to balance, since the Zelda/Sheik player always has more options than normal characters. But all in all, I'll be happy as long as Sheik isn't cut and replaced with nothing.
Not sure how to reflect the fact that I'd rate (Sheik OR Impa) at 100% want easily, but divide it between them. It shouldn't be 100% for both. But eh.
Bowser Jr:
Gonna make this short, since it's a re-rate. Toad's chances have dropped significantly. While swapping out Toad for Toadsworth in Peach's move set (or, less likely, an entirely new move) is still a possibility, and Sakurai's comment makes a lot of sense if interpreted as an oblique reference to Toad, it may just be reading too much into things.
All the Super Mario characters from Brawl have already been revealed, which may suggest that there are more Mario characters to come. But maybe not. I still think we're unlikely to get more than one new Mario rep, but in a larger roster scenario, it's possible.
Bowser Jr chances: 23%
I gave Bowser Jr a 20% chance last time, and at that time I thought Toad was the second most likely after Bowser Jr. I think that Toad's decline helps all other potential Mario reps, but I don't see that it helps Bowser Jr particularly compared to say, Paper Mario or Rosalina. So Toad's lost "chances" don't all accrue to him.
Bowser Jr want: 58%
About the same as before. I want reps for other series more, but Bowser Jr is alright. I don't want Shadow Mario.
Meowth prediction: 13%
Meowth's best chance was a long time ago and I don't think people are thinking he has much of a chance compared to one of the Pokemon being promoted in the new games or the movies. But it's hard to underestimate the numbers here (given that some people think 25% is "minuscule")
Nominations:
Mike Jones x5