Oh the possibilities are endless.
Roxas and Little Chef/Remy. Two characters who went on to become more then what they were born into. Remy became more then just a common rat, and Roxas went on to be more then just a Nobody. Both live in Twilight Town too! Another character would be Buzz Lightyear, since both Roxas and Buzz denied their existences as a Nobody and toy and came to accept it.
Xion is a interesting one. Her character is so bloated with her own lore she can connect/pair with several Disney characters! To piggyback off @palizinhas
with their choices of Stitch, Webby, and Oswald, I would also add in Pinocchio, Tron, Sally, Baymax, any artificially created Disney or Pixar character. But the one I would really love to see her team up with is Hector from Coco. Imagine these two singing "Remember Me" together it's perfect.
Namine and Rapunzel. Both locked away and used for their powers. Honestly I'm disappointed they didn't make the connection in KH3, they even had Marluxia there too.
Terra, Hercules, the Beast. Honestly I would just love to see Terra team with big strong partners like these two. Terra wants to be a Keyblade Master like Hercules wanted to be a true hero. And Beast? I just thought he'd look cool with them.
Axel and Jack Sparrow. Two smooth talking, charismatic trouble makers who ultimately do the right thing. The banter these two could have.
Kairi and Disney Princesses. A easy one. Have the Disney Princesses teach her how to properly use her light powers.
I'll try and think of some for other characters like Aqua later if I can.