Rinoa for sure, so we can get some awesome Squall/Rinoa shipping.
Also, I'd love to see Zidane and Dagger. Dagger in particular would be a good role model for Kairi, I think. Even though Garnet was never too strong (physically, at least), she's one of the stronger FF heroines, in my opinion. And she grows so much in that game! So let's have the two princesses meet (even though Kairi, technically, isn't a princess). And Zidane's just made of win. So enough said. Beatrix could be cool, too.
And ever since I read White Knight by aradian nights on fanfiction, I've always wanted to see some Hope and Ventus interaction.
There are a ton of characters I'd want as cameos, actually (like Shuyin and Lenne for no reason other than I've always wanted to see more with them), but I won't bore you with my long list. However, I will just say one more that I doubt anyone will think of: Llyud from Final Fantasy XII: Revenant Wings. I think they could tie his story to KH in a neat way. For those who don't know, the villain of the game, Feolthanos, stole all of the Aegyl's anima (in other words, their emotions), so the main plot of the game ends up being trying to get the Aegyl's anima back. So essentially, the Aegyl race are a lot like Nobodies.
Anyway...
Llyud... Great catch. Didn't see this one coming.
Also, thanks to Seph0812, I REALLY would love having Sora/Riku interacting with Fran and Balthier, the best characters of Final Fantasy XII.
Also, Zell Dincht from FFVIII would make some hell of an addition, pairing up with Tifa in skills (as both have similar fighting styles).
Others include Kimahri Ronso (FFX, some animal presence aside Fran), Sabin and Edgar Figaro (FFVI, REAL ADDITIONS), Terra Branford (FFVI, AND YOU KNOW WHY), Red XIII (FFVII, our great Nanaki AS A SUMMON), Shelke Rui (FFVII, great anti-heroin), Vincent Valentine (FFVII, another of my favorites), Rinoa Heartilly (FFVIII, AND SQUALL KNOWS WHY), Jecht (FFX, because Tidus needs a father and some exploration), Irvine Kinneas (FFVIII, GIVE SELPHIE A PAIR), Laguna Loire (FFVIII, just to embarass Leon, AND WE DIDN'T HAVE HIM ON BbS), Ward Zabac and Kiros Seagill (FFXIII, Where would Laguna be without his friends?), Zidane Tribal (FFIX, the real monkey), Sarah "Garnet/Dagger" Alexandros (FFIX, why not?), Amarant Coral (FFIX, another one in my list of favorites)...
Also, Ellone (FFVIII) would have some big space, being able to send people into other's pasts. For example, she could send Sora into Ventus' past (and here I am giving a big idea to Seph0812) so Sora could finally understand why Ven's heart resides within his (We know that the idiot shattered himself by defeating Vanitas and the X-Blade).
As for villains, I'm sick from Sephiroth being the only FF villain allowed into the game, and I want:
Kefka Palazzo, the REAL menace
Seymour Guado, as ALMOST EVERY CHARACTER FROM FINAL FANTASY X HAD ALREADY APPEARED IN KINGDOM HEARTS (except Lulu and Kimahri)
Vayne Solidor, as he manipulates many things by the back and could soft-talk the XIII with some ease.
this is going off topic sorry xD
