We all want Marvel content, Star Wars content, and more Pixar content in Kingdom Hearts so badly that we were blinded by how awesome Disney's acquisition of Fox could be for the series.
There are a ton of Fox franchises that could work, but Futurama would be a choice that nobody would expect yet many people would love.
What Kingdom Hearts needs is a franchise that just laughs in the face of the weirdly serious tone that the series has. It would just be funny to see Sora meet some characters who wouldn't be willing to join in on his melodramatic conversations about friendship.
Whilst the characters would be a highlight of this world and having a party of Sora, Donald, Goofy, Fry, and Leela would no doubt be a lot of fun, the potential of the Futurama universe as a setting can't be overemphasised.
Even if the world avoids space travel and sticks to New New York it would still be a lot of fun to explore the iconic locations throughout the city.
Finally, when it comes to boss fights there are plenty of worthy candidates to fight alongside an army of robot Heartless. Robot Devil, Roberto, and Zapp Brannigan would all be great choices for hilarious boss battles.