I have come up with an interesting prediction in regards to certain characters. I obviously don't know if this'll happen or not, but I'm going to say my thoughts anyway:
Some of this is based off of information Loz18 gave us, since we know he had prior knowledge that was very accurate. Specifically regarding the "Heroes Vs Villains" claim he made, which has since been refined into a "Rivalry" theme by the K. Rool reveal. That being said, let's get down to it.
First up:
Shadow the Hedgehog (Sonic's Echo Fighter, Rival to Sonic)
Now, I'm a Shadow fan. I like Shadow, and think he's one of the coolest characters in the Sonic series. That being said, I didn't really want him in Smash. (Still kinda don't, but I'd be okay with it if he did). Since the reveal of Chrom and Dark Samus finally informed people that Echoes work differently than initially thought, Shadow seems super easy to implement. Even more than that, his Assist Trophy—despite being present in two games in a row—has mysteriously vanished and replaced by Knuckles. Why would they just remove an Assist Trophy character like that, but also replace them? I know we've lost Assists before (Isaac, Jill, Ray), but they were never replaced. (Though one could argue that Jill from Drill Dozer became the Drill item). This seems like an intentional decision and one that promotes Shadow to the highest represented level.
This next one is crazy:
The Black Knight (Ike's Echo Fighter, Rival to Ike/Fire Emblem Characters)
Yes, I know; it's another FE character. Yes, I know; it's another sword character. But guess what? I don't really care anymore. We just got Chrom, one more won't kill us. Think about it though, the FE series is now 7 characters strong but none of them could be considered "rival" characters. Honestly speaking, almost none of them even have characters you could consider a rival. Marth, Roy, Robin, Lucina, Chrom, Corrin; none of them are rivals to anyone, and none of them have a character that could pose as a strong rival.
But then we have Ike. Ike, unlike the other characters, actually has a strongly established rivalry that lasted not one but two games (which is already more than the Smash FE characters have even had in the FE series, save for Marth). Black Knight could fit the stronger/slower/armored/heavyweight/swordsman archetype that Smash does not have yet. Also, canonically speaking within the lore, Ike and the Black Knight legitimately share the same fighting style, which makes being an Echo not only a good idea, but most likely the optimal idea. Also, the Black Knight was a Mii fighter costume in Sm4sh, so if that boosts his chances...
Gonna go the opposite of most people's expectations with this one:
Akuma (Ryu's Echo Fighter, Rival to Ryu)
As far as I've seen, most people would insert Ken here as the obvious Echo Fighter choice for Ryu. I decided to go with the road less traveled with this Echo Fighter choice. Yes, I know that Ken is very much beloved in the SF series, but I believe that Akuma brings more to the table and is equally loved if not more so. He's visually more distinct than Ken, his presence commands more attention than Ken, and honestly he's more of a rival to Ryu than Ken will ever be. True Rivals > Friendly sparring partners. I'm not saying Ken is a bad Echo choice, in fact he's almost inexcusably one of the better Echo fighter options available. But if Sakurai had to choose between Ken and Akuma, I think he'd go with Akuma. (Also, Raging Demon Final Smash. 'Nuff said.)
And let's finish off our Echo Edgelords with something we can all see coming from a mile away:
Black Shadow (Captain Falcon's Echo Fighter, Rival to Captain Falcon)
Marth, Link, and Mario all had the distinct honor of being the progenitor to two other characters (one of Marth's even has an Echo of their own, now), so it's Captain Falcon's turn to receive that honor. Honestly, this accomplishes several tasks: More F-Zero characters, check. Another villain character, check. People actually want Black Shadow, check. Rivalry theme criteria, check. Honestly, there's no reason not to have Black Shadow appear as Falcon's Echo. He could have stat differences, or he could be exactly the same as Falcon. Either way, it wouldn't matter to most of us. He would be a meaningful addition to the roster, one people would actually be happy with.
Now, to start off my non-Echo/non-edgelord characters:
Geno ( Third-Party Mario Series)
Now, this is a personal want of mine. However, I also know that this is a personal want for Sakurai as well. As far as we've been told, Sakurai has had 3 characters that he's wanted to implement as playable characters for a number of years now: Takamaru, Simon Belmont, and Geno. Takamaru, due to not having enough western popularity (despite coming to the west, the Wii U did so poorly that I still doubt people would recognize him) couldn't make it past Assist Trophy status. He did get a Mii costume though. Simon Belmont, however, was recently revealed as the next third party character with an exceptional amount of love being poured into his series, even to the point of giving him a massive music track list and an immediate Echo Fighter in Richter. Skipping far beyond Assist Trophy status, Simon pretty much cleared the way for Geno to be added to the roster.
Geno has been wanted by Sakurai, and many fans, since Brawl. Implemented as a Mii Fighter costume in Sm4sh's DLC when Cloud was added, Geno has a shining opportunity to follow in the footsteps of both K. Rool and the Inklings and get promoted from Mii costume to fully-realized playable character. He's niche and though he's very beloved, he isn't relevant, but neither was K. Rool and he made it anyway. In Ultimate, relevance doesn't matter. It seems a character's star power actually means something again. To top it all off, he'd bring the Mario series to a dominating 9 characters (this is counting Pokemon Trainer as a single character). Hilariously, everything that needed to happen for Geno to appear has happened: Cloud bridged the gap to Square-Enix, Simon joined as one of Sakurai's personal wants for the game, and K. Room destroyed the relevance argument. This is Geno's best chance.
And to end off my predictions:
Monster Hunter (Third-Party Monster Hunter Series)
We all saw the Monster Hunter stage with the Rathalos Boss, and the Rathalos also doubling as an Assist Trophy. I know the the major focus of the MH series is of course, the monsters. However, it also boasts a strong focus on making your character powerful enough to crush said monsters. It's not like Pokemon or Yo-kai Watch where the monsters are almost exclusively the focus. The hunter's themselves share half of the focus, hence their half of the game's name: Monster Hunter. That being said, Capcom acknowledged the Hunter in Marvel Vs. Capcom: Infinite as a DLC character, and the Rathalos Armor set was included in Sm4sh as a Mii Fighter Costume, not to mention all the Nintendo/Monster Hunter crossover content these last few years. I legitimately believe this is evidence enough for the Monster Hunter to appear as a playable character.
And those are my predictions. These aren't just my wants, these are what I feel will actually happen in Ultimate, because this game destroys a ton of previously-thought precedents. If this is the game to outdo them all, then let's blaze forward at Mach 20.