I'd say Sora, Riku, Roxas and Axel.
Sora would fight with the Kingdom Key, and he could use weakened versions of his Magic Spells as long range attacks, but he does not have cure magic, to allow for a balance in battle. his Final Smash transforms him into one of his Drive Forms from KH2 and deals massive damage on his opponents. The way the Final Smash works differs depending on the Drive Form.
Riku would fight with the Way to Dawn, and he'd be able to shoot Dark Firaga-balls. When he and Sora are on the same team, both of them become stronger and are capable of dealing a Team Attack. In this attack, he and Sora basically mimic their movements and attacks in the Limit Move they are able to use in the World That Never Was. His Final Smash involves his clothes temporarily changing to an updated version of his Darkness Suit from Kingdom Hearts 1, and for that time he can wield the Keyblade I like to call the Heartbreaker (the Heart-Unlocking red-and-black one), and he does that attack where he warps around the level and rushes his enemies.
Roxas fights with the Oathkeeper and Oblivion Keyblades and uses most of the attacks seen being used by him in the Sora Vs Roxas battle in KH2 FM+. He can fire bolts of Nothingness for a long-range attack. His Final Smash makes him rise into the air, glow, and then lots of Nobody signs rush around the Arena slicing up his enemies.
Axel fights with his Chakrams and can use them for close to long range, and all his attacks are infused with Fire. When fighting alongside Roxas, they both increase in strength and can perform a Team Attack. His Team Attack with Roxas would involve Roxas' Keyblades and Axel's Chakrams lining up and creating a massive tornado of of Fire and Nothingness. His Final Smash produces massive walls of Fire on either side of him, and they would slowly decrease in height and move away from him. The walls would descend and completely dissapear once they reach the ends of the Arena.
If there are to be any more Kingdom Hearts Character thrown into the mix, I'd have to say Kairi, Xemnas, Master Xehanort, Darkness Soldeir, Terra, Aqua and Ven. That's about it for exclusives, and I doubt Disney or Buena Vista Games would allow Micky, Donald, Goofy, or any other Disney Characters to be used.
The only way that any of these Characters could possibly be made availabe is via download.