What's interesting is that Stella also had a near death experiance that let her see the spirits of the dead as well. He might be in the Final World searching for her Verum Rex counterpart. That could tie somehow into Kairi who died and will come back to life.
"The legend stated that whenever Etro opens the Unseen Gate, the souls of the dead ascend as a streak of light to the heavens and through the Gate. When few among the dying see the light, they are given power from the Kingdom of the Dead. Stella stated she can see the light, as could Noctis." - From the FF Wiki
So when Noct and Stella had their near-death experience and survived, they were given power. This is implied to be both their Armigers and the ability to see the light of expiring souls.
Let's say Yozora = Versus XIII Noctis for the sake of the discussion. If that's the route we're going with, Versus XIII's lore could easily be implemented into Kingdom Hearts. Kingdom Hearts is said to be the brightest Light in existence, so bright that people fought over the light (from what the old fairy tale says). And we know Xehanort tried both in BBS and KH3 to access Kingdom Hearts in order for his own gain. Well... Since we don't know what it actually is besides "light" , what if somehow seeing the light from Kingdom Hearts itself grants you a certain ability? Or interacting with it somehow? Especially since in a way, Kingdom Hearts could very well be a non-conscious deity.
And, it seems that when people die, they get sent back to Kingdom Hearts (I'm not sure about this, I need to double check) -- similar to how in an early Versus trailer, you could see the light of an expiring soul going to the Unseen Gate. If that's the case, you could pretty much swap out Etro for the Kingdom Hearts moon and there's not much of a difference. And if The Final World is a place of purgatory for those who still hold on, maybe somehow The Final World and Kingdom Hearts are connected. Not necessarily the same location, but... Maybe similar in some ways, or the function of The Final World correlates with Kingdom Hearts itself.
Up until the Secret Ending, we were under the assumption that Yozora was a fictional character in a video game in Toy Box. But with the Secret Ending and the new trailer, there's more to it that's going on. Yozora also doesn't have the typical look of someone that belongs in the usual KH Universe, he looks like a character straight out of a Final Fantasy game. Somehow though, he and Sora are meeting in a place that seems to be The Final World, but darker (which implies time is a thing here). If Yozora is looking for his "Stella" in The Final World, and he's able to meet Sora somehow... Another idea I have is, what if The Final World is a place where jumping to different worldlines occur? And that's why Ven's Chirithy is there?
Plus, Chirithy did mention that Sleep and Death are closely intertwined, and in KHX, we didn't get to the Unchained Realm until Player was seemingly dying (or just losing consciousness due to exhaustion) in the Keyblade Graveyard, and the Unchained Realm is apparently the Realm of Sleep (I think). So maybe the key to crossing worldlines is via Sleep or Death.
Tying it back to Versus XIII, Noctis would sleep a lot and his dreams were linked to a completely different reality with real-world ramifications. Platinum Demo for FFXV comes to mind, when Noctis as a child lost consciousness due to being mortally wounded, and Carbuncle had to guide him to Noctis's safe place (the Regalia) in order to wake up. And, Lucis is a kingdom that reveres death (in v.XIII they even had reaper motifs), Insomnia obviously being related to sleep in name.
So, if Yozora is supposed to be "Noctis" (again, for the sake of the discussion lol), maybe him appearing in The Final World is because he's either sleeping
or like you said, he's searching for "Stella" (her Verum Rex counterpart) after sacrificing himself somehow. Or another possibility, he has the ability to reach The Final World with his powers at will, and maybe his different colored eye plays a part in it somehow (lol Rinnegan). He's looking for "Stella", hears someone's voice (Sora's in the trailer, because he's calling out to someone in The Final World), and ends up seeing Sora.
All of this is a
huge stretch, but I feel like a lot of the Versus XIII references in KH3 are done for a reason (besides the justified salt lmao). If nothing else though, there's clearly enough of Versus XIII material (that was made public knowledge) that could easily be repurposed into Kingdom Hearts. Other than the reaper worship and the blood you'd see in some of the trailers, most of it could definitely be a game that fits within the franchise. Or, it'd be the first KH game rated T for Teen. Either way, I think it can be done.