I don't have predictions. What I have are insane, incredibly implausible dream scenarios that have no chance of happening but damn if it wouldn't be cool if they did. So I'm indulge in reai world fanfiction for a minute:
KH3 doesn't just get a trailer at E3, it makes new gameplay/world/character reveals at each of the 1st party presentations. Yes Sony, Microsoft, & Nintendo all getting KH3 love!
Starting with the XBox conference, KH3 is 1 of the games shown to demonstrate the power of the Scorpio. No one's given a damn about KH3 being an XB1 title until now & now the time to change that. The theme of the XBox trailer is This is Kingdom Hearts 3 at it's very best. Because afterall the Scorpio is by default the most advanced system that the game will be released on. The trailer shows the Toy Story world, with visuals nearly on par with the movie itself. Sora, Donald, & Goofy are turned into a Gundam kit model, rubber duck, & a wind-up toy dog respectively. Pete shows up near the end as a large plush doll. New weapon drives & attraction flows are introduced. Everyone leaves happy & content thinking they've all there is for KH3 this year.
Then the next day Sony blindsides them all with even more KH3 reveals. The theme of the Playstation trailer is We're large & in-charge & we've got the coolest shit! This is reinforced by this trailer showing the 1 world that would get more mainstream attention than any other: Star Wars. But oh, there's no Sora here. For this is where Square finally drops a bombshell they've been hiding up til now: there are other playable characters. We see Riku & King Mickey. trudging the deserts of Tatooine, battling storm troopers & new heartless along the way. The quick shots of the duo meeting Luke & R2 & them getting on board the Millennium Falcon get crowd so hype up they don't even fully notice at first that the Riku gameplay is CoM card-based. Yes, it seems each of the playable characters has completely different battle systems, with the purpose of covering the series history so that KH3 feels ever more the culmination of all that's transpired so far. Mickey& Luke are present as on-field party members but friend cards activate team-up moves.
At this point there's some speculation on various forums about the possibility of yet another reveal during Nintendo's direct. However most people will ultimately write that, believing that the tablet wouldn't be powerful to run the game & after Riku & Aqua there's no other viable characters anyway. Well suffice to say KH3 on Switch is confirmed, though the footage shown isn't running on the actual hardware yet since the full support for Unreal 4 only just now became available. The Switch trailer's theme is Hey um, we can play too guys! We are shown Kairi with her new party members Minnie & Daisy on a snowy hilltop looking at the ice palace of Elsa off in the distance. Rapid-fire scenes showcase the story from Kairi's perspective: her training in the hyperbolic time chamber, What happens to Lea after finishing his training, the Impetus of her new journey, the introduction to her new party members. Gameplay shows her battling in Twilight Town, Corona, & Arendelle. Her gameplay is the Command Deck. The big highlight of the trailer is the boss battle with Marshmallow, with flowmotion antics happening in-front of, in, on, & all over the ice palace as the monster pursues her. The hinting of another bossfight with Elsa serves as the stinger.
All three characters are going to be playable in all versions of the game, there's no platform exclusive content. Square just wanted to each hero ample time to show off & presenting at all 3 events was deemed the best way of doing that. The HD remixes are all announced for XB1 & Switch for late 2017, to give those players something to tide them over until KH3's 2018 release.
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What's actually going to realistically happen? IDK, Lego KH maybe?