Alrighty, time to bring out my opinions on the characters.
-Palutena: Like I always said, she is the one most likely to be the 2nd rep if the KI franchise gets one. To the best of my knowledge, she practically is the only one besides Pit who's been in and played major rules in all 3 KI games, that of which not even Medusa was, who was absent in "Of Myths & Monsters". Then also being the one to appear in Brawl, as a mere image of Pit's Final Smash, and a trophy, not to mention the one Pit SSE cutscene which wasn't much, but she was there. The potential escalates with KI:U, which has her as a boss twice, with potential moves she can use, but that isn't all she could be limited to. She could obviously use some moves from her boss battle, but also make use of improved versions of the "Powers" she let Pit use (Like Reflect Barrier), and let's not forget her staff that she can use to whack others with. She could shapely become a tall, semi-strong, semi-heavyweight character whose style is defensive and zoning while making use of projectiles.
She does have her chances for interesting moves that some of us probably would never thought of. I mean look at Rosalina having interesting moves, including that galaxy thing appearing from her dress (intention to be hilarious). Imagine what Sakurai could make Palutena have if she gets in. But like I said again, she has the highest chance to be the 2nd KI rep, whether some want to deny it or not.
-Medusa: While it is true that she would be interesting and we've yet to have a female villain, implying that there's boss battles in this game, I kinda see her as a potential KI boss representative (if not Twinbellows) rather than a character, but that's just me. I won't mind if she's in, but I will be angry if she makes it in and not Palutena. As for the Final Smash should she get in, there's always detaching her head to become the classic monster Medusa and having that dark laser breath like in the battle but what do I know?
-Dark Pit: Gonna say no, no, and no. However, I wouldn't be surprised if Pit's previous black-colored alt costume from Brawl was updated to look like Dark Pit by actually having the right-colored Laurel crown and eyes. Other than that, not a separate character. It's not that I don't like him much, it's just I ain't fond of identical characters getting in to waste spots, like with Melee having clones, mostly noted of what I mean, Dr. Mario and Pichu, though the former annoyed me more.
-Magnus: I just don't like him too much, and we only get to see him a few times. Besides, he'd kinda remind me of Ike too much, without the fire. As one said before, he isn't too important to the KIU story much I think. So I'm not for him.
-Phosphora: As much as I like her, and that she's likely built to be a shocking electric-based fighter, she isn't that important to the KIU story either, doesn't even appear as much as Magnus, despite her to be more interesting than said swordsman imo. But like I said, I like Phosphora, but she really doesn't have much chances at all. Maybe as an Assist Trophy at best.
I rather not talk about the other characters for different reasons, including some that I feel are impossible to be in and such. As for Viridi, there's that 3ds stage of Chapter 11 with the Reset Bomb Forest. I assume she'll be in the background after the explosion just like in KIU.
Took a while to type it all and think, but I hope it helps.