When Kingdom Hearts came out it was a breath of fresh air, but with the first arc of the series reaching its conclusion, there are a lot of characters that didn't make it into any of the mainstream titles. Despite being a collaboration between Square Enix and Disney, Kingdom Hearts, as a series tends to delve more into the Disney side of things while having a limited selection of Final Fantasy characters. Kingdom Hearts 3, in particular, had a marked absence of Final Fantasy characters compared to the previous titles.
Fans with a keen eye will realize that the only FF characters that made it into the KH franchise were the ones Tetsuya Nomura, the director, personally designed. That means while Cloud and Squall made it in, future series protagonists were left behind. Here's a few Final Fantasy characters that would've meshed with Sora really well in his newest entry.
Zidane Tribal is quite possibly the cheeriest Final Fantasy character in the older entries before the jump to PlayStation 2. The world of Final Fantasy IX, Gaia, is stunning and includes a plethora of fun locations. Zidane is the thief of the party and is probably best compared to Peter Pan from the Disney side - juxtaposing the two could have been really fun. Zidane isn't an entirely one-dimensional character, either - he carries a lot on his shoulders that Sora would love to help with.
FF12: Fran and Balthier
Fran and Balthier from Final Fantasy XII could be said to be the real protagonists of the game, as Balthier calls himself the "leading man" while Vaan mostly tags along for a good portion of this entry. Replacing Vaan with Sora would be easy, and it would be simple to have Sora trip on a switch during a heist and cause trouble for Balthier and his partner. The bosses lend themselves well to Kingdom Hearts 2, as well - Sora fighting a demon wall as it closes in to crush them would be easily programmed and make for a great moment.
Lightning from Final Fantasy XIII is all stoicism and takes everything seriously, which makes her a perfect choice to team up with Sora. Lightning carried the Final Fantasy XIII trilogy and has become a gaming icon, so Sora could do worse than following her and trying to make her smile with Donald and Goofy. Come on, Lightning: This boat runs on happy faces.
FF15: Noctis and Friends
Noctis supports the cast of Final Fantasy XV and doesn't exactly work without his team around him; it would be an awkward road trip without Ignis, Gladiolus and Prompto. Simply stick Sora in the car, have him help collect one of the Royal Arms and culminate in a boss fight with someone from Organization XIII who got the weapon first.
It's a shame Final Fantasy was so underrepresented in Kingdom Hearts 3. Previous titles included fan favorites such as Auron, Squall and Cloud, but Kingdom Hearts 3 dropped those characters and just used Final Fantasy names for objects in the world - Gummi Ships, for example, were named after the summons from the series. Hopefully Kingdom Hearts 4 can take from the amazing wealth of source material and add it in, instead of giving us a shot by shot recreation of Let It Go to listen to again.