When Nomura said that there won't ever be a final fantasy world, I initially was a bit disappointed, but realize as well that this is probably for the best. I think it is very different turning a film, in general, into a video game than it is turning a video game into a...different video game. It's just a bit strange.
Now, if they decided to do a "world of final fantasy" type thing with some final fantasy "hub" as a "world" then, sure that might work out okay. But having a "world of Spira" for example? Bad idea. For the main reason that, if they did something isolated like that for all the FFX characters, you would then establish that this is where the FF characters for that game would have to come from. This would basically nix them from appearing in any other place unexpectedly which is honestly half the fun of having the cameos in the first place. If the ffx characters all have to be from "spira-land" then it would be weird to run into them in some future game at, say, radiant garden or something. (ignoring, of course, that wakka, tidus, auron, yuna, rikku and paine all have already appeared before and have established roles...)
like I wish they would put monsters, summons, airships, and Chocobos from the Final Fantasy series into the Kingdom Hearts series.
Well, technically I still argue that Parasite Cage in Monstro is basically a cameo of Prison Cage from ff9, I agree there should be more subtle cameos in terms of heartless designs. They have done a good job of implementing some fun little shout-outs in the gummi ship models at least. And speaking of gummi ships, that's why I don't think they would really bring in airships ever (unless it was another gummi model). But I would be down for chocobos somewhere.
Personally the one summon I want to make an appearance is Carbuncle because he's always been my favorite and I think he would fit in just fine into the KH aesthetic. Though I know through hacks they were able to figure out that Bahamut was at least on the cutting room floor at some point for the first game. So there's a possibility they will eventually put it in.
However I still wish that Square would put more elements from the Final Fantasy series into the Kingdom Hearts series besides just the characters,
I did just want to point out, though, that there are plenty of FF elements other than the characters. I mean, from the very first game the reason they have the "command box" interface in the bottom left corner is to mimic the old atb turn-based style of older final fantasy. That's also not mentioning the magic system and items all using FF terminology. BBS's ability acquisition method is the exact same style as FF9, and in the coliseum Sora mimics quite a few victory poses from other FF characters (Squall, for example). One could even argue having "summons" in the first place is because of it being a FF troupe. Quite a few of Sora's move sets were named after FF terminology (Zantetsuken, for example). And in a few of the handhelds we saw the appearance of a few status ailments/spells straight from the FF black mage handbook like poison, sleep, bind, etc with esuna coming hand-in-hand to save the day.
So, from a gameplay perspective alone there are quite a few elements of final fantasy sprinkled in like a nice little spice. KH obviously functions in its own way, but it absolutely has drawn from FF as its clear origin source in ways beyond just throwing some cameo characters in.
Also, it is worth mentioning nearly every FF has an element of "friendship/teamwork" that is played up to a greater degree in KH, but I can see quite a lot of the same themes repeated in both series'. FF4 in particular meshes very well plot-wise with KH in the struggle to understand the line between our personal darkness and light. To me, they basically took the themes from FF and stuck them in a Disney playspace and came up with something unique and wonderful, but the ideas were definitely spawned from a FF egg (if that makes any sense).
Without Final Fantasy, not only would KH likely not play the same, the themes might also have ended up being vastly different, with the game being undoubtedly much less complex than it is now. If we removed FF from KH it wouldn't just be erasing a few character cameos, you would be losing much more.