While ultimately it probably doesn't matter since at this stage a crossover is unlikely, I still don't think it would be a problem at all or any more jarring compared to what has ever been done before or what will be done in the future, even from a general tone perspective. XIV often deals with heavier topics, yes, but it isn't one to shy away from general light-hearted things now and again and that is one of the reasons I love it "malms" more than any other MMO out there. It's serious but it's plenty goofy.
There is a reason why the Manderville sidequests are massively beloved by most and I don't think I have ever heard anyone say they don't fit into the general atmosphere of the game. Like...when you have a nearly naked man wielding a tiny golden hammer pile-driving his son from heavenly heights into the dirt followed by a jazz-dance number (with a zombie as a guest character) and have that be completely 100% canon I am sure that having a few KH characters appear to have an optimistic chit-chat about friendship would be tame in comparison.
And even if the "general" tone is heavier in XIV, crossover events have always been kind of...weird and non sequitur. For example, the current Yo-Kai event they brought back that is ongoing now is just like "hey...here is this guy with funky hair who just sort of showed up to showcase his roving band of goofy ghosts and he wants YOU to play with them! Whoopie!" Like...Yo-Kai couldn't be further removed from XIV in terms of theme since it is literally a children's pokemon rip-off game. I guess because you could say that some of the Yo-kai are dead or based on creepy mythology it deals with death...maybe. And while yeah, it is weird that Yo-Kai is in XIV and does sort of clash thematically, they found a way to make Yo-Kai work so why not KH which is even CLOSER to XIV thematically.
Even the FFXV event was really weird in concept. Noctis just sort of...shows up through a rift in time/space with his car and you fight garuda while you wait for his car to get serviced and then he leaves. It was the most random thing ever but it was still pretty cool. Most people just rush through the dialogues for the goodies anyway.
Gunbreaker would be a perfect class to give a Keyblade considering the stance of your character has them holding it over their shoulder like Sora when you run around and has as ability named "heart of light". Giving us costumes and clothing glams from KH or even just some Orchestrion rolls would be pretty jam.
Also...just wanted to point out that while this is unconfirmed by any means, I have always been INCREDIBLY suspicious of the black mage weapon in the Gold Saucer named "Heartless" that is, essentially, a giant golden KH style heart on a stick. To me that is almost like a little subtle nod they have already put in the game towards the KH series.
Even though largely this discussion is moot because it's not gonna happen it looks like, I mainly just wanted to say that FFXIV is campy as all hell at times and no one can tell me that that camp doesn't match KH to a T, haha.
