The soon to be most hyped up characters ATM are IMO are:
1. Sora: Feels destined at this point. I think it's time. And the hype for Sora if he's announced will be in the stratosphere for Cloud; don't be mistaken about that.
2. Banjo: Don't see it happening, but the hype may begin to ramp up for this. If it happens, it will be DLC and hit us like a thief in the night, similar to what was experienced in the Cloud reveal. I expect the noise to really pick up though as we progress and more and more newcomers (so freaking weird there are no veterans left to confirm; this feels like an alternate reality) get confirmed.
3. Shadow the Hedgehog: I think it's happening, and I think many people are going to be hyped it is happening.
4. Dixie Kong: A bit quiet now from the K. Rool hype, but expect the hype for Dixie to ramp up. She would essentially "complete" the Donkey Kong Country series roster.
5. Heihachi Mishima: After the tease of an appearance in Smash 4, I think this is just a hand-in-glove fit, and I am reasonably confident it will happen.
6. A new Pokémon: Always a big deal. Whether Incineroar or Decidueye, it's gonna be a big hit.
7. Rayman: Not sure of the odds here any which way, but the hype train from the fake is chugging along slowly, and may begin to pick up steam soon.
8. Rex: I think the hype train is forming with this one. He's gonna get the train rolling for JRPG fans.
9. Ken: 4 year old me is screaming at this being almost a lock, and so is adult me now!!!
But really, Ken has a huge following; expect it to get loud for Smash!
10. Spring Man: Not the biggest fanbase, but we know it's happening. I expect people will warm up to it.
~Honorable mentions~
3rd Party:
-Mega Man X: Once people realize he can actually happen as an Echo Fighter (only) and has a very realistic shot, oh man... it will get rolling (same goes to Proto Man, though I imagine to a lesser extent).
-Big Boss/Raiden: Same but not as realistic a shot as an Echo Fighter.
-Sephiroth: If people think there's even a small chance Sephiroth is an Echo Fighter, RIP human existence, because this hype train is gonna run over speculation!
-Phoenix Wright: This hype train hasn't even been loaded yet.
-Geno: Still stuck in the "cult zone" and not quite the "mainstream cult zone," as well as being overshadowed by another (Sora).
-Lloyd Irving: Same, albeit with a bit more to it (and Heihachi's shadow instead of Sora's).
-Monster Hunter: The hype train might get rolling, but I don't see much to it.
Nintendo-owned:
-Impa: Echo Fighter possibility is slow to pick up.
-Isabelle: We don't know if she will be an Echo Fighter or original moveset character if we get her. Both are in play, but irregardless she is rather popular.
-Fire Emblem: The Three Houses Main Character: Not even confirmed yet, but once it is... expect the Fire Emblem hype train to get rolling for DLC.
-Black Knight: Hype train for this Ike Echo Fighter candidate could really reach a fever pitch.