To me, this just seems like SUCH a perfect fit for a potential crossover event, I feel like I barely even have to justify it.
Well, the series just reached its 20th Anniversary -- another reminder that we are all marching towards our inevitable mortal end -- and honestly, I think people would accept almost ANY excuse to do something with Kingdom Hearts.
Personally, while I certainly don't claim to be any kind of an expert on KH lore, I would probably pick something along the lines of having a "crossover" with the Keyblade wielders during the Union Cross (or whatever it's called) mobile game. Maybe a portal of some kind opens up, transcending both time and space, where we deal with some Heartless, and interact with some friendly Keyblade wielders.
I suggest this route, because I feel like interacting with Sora or the gang feels like it would maybe be TOO big of a crossover. Plus, I think it's important that there are a LOT of Keyblades floating around, because I would imagine Keyblades would make some VERY enticing rewards!
My thinking is, give each Job in FFXIV a different styled "Keyblade" of their own to pursue.
However, this obviously leaves some highly questionable choices, as not all jobs are clearly even CAPABLE of wielding anything remotely resembling a Keyblade.
Similarly, I think some Keyblades are simply "too important/iconic" to be usable by anyone OTHER than Sora or the crew, and I'm going to avoid using any "franchise-specific keyblades", instead sticking to only those designs born from the KH world specifically (or a few Final Fantasy references).
Of course, you COULD get away with simply having the classic "Kingdom Key" as an option, but I feel like that's very much an all-or-nothing thing. It's clearly the most iconic, so it would have to be like Edda Blackbosom's scythe, where you have the same weapon available to multiple jobs, but not EVERY job has access to it (PLD, DRK, GNB, and SAM are the only ones that strike me as easily being able to pull off wielding the standard Keyblade).
With all that in mind, here are the weapons *I* think would suit each job...
* = Not a keyblade
PLD - Oathkeeper w/ Save the King (KH2 design)
WAR - Fatal Crest (alternatively, Saix's claymore)
DRK - Oblivion
GNB - Sleeping Lion
* WHM - Save the Queen (KH2 or KH3 design)
* SCH - Namine's sketchbook
AST - Two Become One w/ Luxord's cards floating around (the Organization XIII logo in place of a "globe" floating in the middle?)
* SGE - Gummy Ships
* MNK - I dunno, man. Xemnas' hand-lightsabers?
* DRG - Xaldin's lance
* NIN - Larxene's kunai
SAM - Guardian Soul or One-Winged Angel
* RPR - Marluxia's scythe
* BRD - DANCE, WATER, DANCE
* MCH - Xigbar's gun
* DNC - Axel's weapons from KH2
* BLM - Mage's Staff
* SMN - Zexion's book
RDM - Spellbinder
* BLU - Dream Rod
...holy crap, that actually took a lot of time to look up and think of fitting examples.
Anyway, for a collectible Minion, you absolutely, 100% MUST have a little Heartless minion for us to get. Those little guys are just adorable.
I'd personally also really dig an Orchestration for the Gummy Ship music from KH1. Or Traverse Town, but that might be getting carried away (KH is full of amazing music, so hardly an issue finding great songs, assuming they were ALLOWED to have them in FFXIV).
Of course, it would also be very cool to be able to acquire a Mount of some kind as well, but honestly not quite sure what you would go with. I feel like a Gummy Ship would just look "bad" to anyone who ISN'T a Kingdom Hearts fan... but then, we got the Whisper-a-go-go mount, and that things looks real dumb to me (not even remotely familiar with Yokai stuff). Can't really think of any alternatives, though.
Anyways, for the actual CONTENT of the crossover, as much thought as I put into the possible REWARDS for it, I think you could actually go quite simple. I think you just add some FATEs all over the place, with Heartless pouring in, and maybe a chance to a Darkside Heartless to appear at the end. A little quest-chain in which you help out a Keyblade wielder, enough they deem fit to make you an "honorary Keyblade wielder", permitting access to unlocking said rewards.
Of course, if it's a crossover event with NO rewards (or at least no Keyblades), you could just make it as simple as "thanks a ton for helping with the Heartless situation in this world, who knows, maybe one day you might even be chosen to become a Keyblade master", just to keep the dream alive.
...this is obviously all HIGHLY unlikely. But man, it would be SUPER satisfying!