The fact that Palutena actually got in Brawl alone makes her far more likely than any other KI candidate IMO.
She was like the 7th Nintendo character you met in the game.
You met her before major playable characters like Link, Samus, Pikachu, Luigi, Bowser, Donkey Kong, Marth, Dedede, Red, Ness, Ganondorf, Yoshi, etc.
I bet a lot of people didn't finish the Subspace Emissary, but who didn't make it this far? This was only the 2nd level!
Due to her being in every KI game, a prominent appearance in the most recent game, and being known by SSB fans who played Brawl, I find Palutena to be the most likely newcomer out of anybody in any series. (That is, counting Mewtwo as a veteran and not counting Little Mac due to his confirmation)
Nobody else has a long-lasting leak. She is the only one. (Not talking about BS text leaks that get deconfirmed every other day)
And not only is she likely, but she has enough popularity and requests that could get her in without all of this.
She also has the recency (like Villager, WFT, Rosaluma, and Little Mac) and uniqueness (like Mega Man, Villager, Rosaluma, and Little Mac).
She wins in every category. She even adds an iconic female. Which there are really only 2 left.
You may think she's likely due to uniqueness potential, you may think she's likely due to fan popularity and large amounts of requests, you may think she's likely because of multiple appearances in her series and recency due to a prominent appearance in Uprising, you may think she's likely because her name is in the series' title in Japan, you may think she's likely because she was in Brawl, as a Final Smash and a part of the Story Mode, you may think she's likely due to Sakurai's deep involvement with her character and the series in general, you may think she's likely due to statements made about female additions by Sakurai himself, you may think she's likely because you believe the leak is true, you may think she's likely because you think Kid Icarus deserves 2 characters, or you may just think she's likely because everybody else does.
But I think it's all of the above!!! :3
Is there a single character you can make such a strong argument for other than the green-haired, lovely Goddess of Light?
I don't think so.
I see this happening with Kid Icarus characters.
Playable: Pit, Palutena
Assist Trophy: Phosphora, she can still possibly be the Assist Trophy.
Stage Cameo: Viridi
Pit Color Swap: Dark Pit
Palutena Color Swap: Medusa
Trophies: Pit, Palutena, Phosphora, Medusa, Hades, Pandora, Viridi, Dark Pit, Fighter, and maybe a weapon or two.
Sticker: there is no stickers.
So you don't think there will be any bosses from Kid Icarus? That could happen. I just hope we get either Hades or Medusa as a boss, I could definitely see one of the 2 happening... I see it in my head Pit and Palutena fighting a giant Medusa in Smash Bros Boss Battle Co-op mode... yes...