Okay so my (tentative) ideas for the KH/PM crossover so far are:
-this does not overwrite KH lore, but adds to it. Likewise doesn't completely overwrite PM lore, but would cause minor canon divergences and differences
-the Darkness would have been attracted to the world (the City) by two, technically three, things.
first - the fact that the City is a place full of darkness, ready for everyone to turn into Heartless
second - the way that Ayin's amnesiac self went by "X" during the loops. my dude accidentally giving himself the Recuscant's Sigil there, oops.
this is where it gets funky, because I was thinking "okay but what woud be a break-off point from PM canon that wouldn't literally break the narrative several ways to Christmas" and threw several ideas out (WNDD? nah, several reasons. during loops doesn't work either. not sure about after it all, but that's somewhat better) and eventually, I decided on-
third - Argalia and his Reverberation Ensemble open the Door.
So, and this is because I have no plans on starting another great epic fic just yet, I'll summarise the idea.
Argalia opens the City to the Darkness.
No one is prepared, and no one has a keyblade, so the Heartless can't be held back. The closest they have are Angela and the Sephirot, who were all exhausted after a long fight.
When Angela looks around for Roland, he's already gone, taken by the flood of Darkness.
Now, I'm thinking that somehow or other, Angela gets lost out in the worlds and has a grand old time being torn between "yay I can explore and I'm free of the City and I'm free of everything" and "how much of this is all my fault" and "when everything's back to normal, if it ever is, will I ever have Roland back as my friend again, will he ever come back, even if he never forgives me."
I also have the thought of Ayin somehow getting spat out because a) the whole idea of a crossover started because I wanted to give him a keyblade, and b) if we take the idea that the Keter trio were all parts of him and were then put back, and the Light is all scattered... that means the full being that is Ayin had to go somewhere.
So. Somewhere around KH1 or 2, we get a "human" Angela wandering around with a very conflicted attitude, and an Ayin who's in Traverse Town.
As for the Sephirot? I'm kinda thinking that since they're constructs of the Light it's less easy for them to just be re-formed outside of the Library, and also just. Maybe it's self-indulgent but it's my idea, so- Ayin, as the original (and with Roland MIA and Angela also missing and not fulfilling the role, currently only available) Keter, the Sephirot are just... hanging out in his heart. Like Ven and Roxas and the others did in Sora's.
I can imagine it being a kind of strange thing given there's so many of them, but hey. Whatever works. But also the idea of Ayin constantly feeling Ben/Hokma being encouraging and warm in his heart, Chesed giving him faith, Netzach pushing him to keep going...
Ayin's keyblade would be made of his bonds with all of them, and resemble the tree that the Library looks like on the outisde.
He'd... also be researching ways to put a soul back into a human body. Which would coincide nicely with Ansem's own research, and what happens in KH3 with Sora needing to vacate his own heart hotel.
So... with that in mind, I'm thinking of Sora fixing everything up and the City getting put back to "normal" back the way it was, Angela and Roland finding themselves back where they were, Roland having his crash-out, Angela choosing to put all of the books back (especially after her own solo adventure), and the Distorted Reverberation Ensemble being basically the very last Heartless before the world's shut off from the spaces between (at least for a while).
The Sephirot are back where they were before in the Library (with Hokma especially feeling rather smug and like he's just had the best experience ever and wistfully wishing he could go back). Ayin, meanwhile, is now out of the light and stuck in the City without a place to go, and unfortunately for him the ability to wield a keyblade doesn't translate into anything useful in the City... oops.
Dante, some months later, having no idea what this key sword thing is. They had it when they woke up. Sometimes weird dark things attack them and the Sinners and the Sinners have weird key sword things that none of them say they had before they had their contract with the Manager, so that's weird, right? That's weird.
(In other words, they ain't got no memories of it because they're amnesiac, and their ability to use a keyblade is transferred through the contract, but the Sinners' keyblades are more like... cheap copies, rather than the real deal, until they manage to unlock their own.)
Honestly? This is the kind of crossover AU I'm probably just gonna talk about and doodle for rather than make an epic fic just because that fic would take literal years to write, hahah.... but hell if it ain't all fun.