I'd love to see the return of trinity's from KH1, reaction commands and control scheme from KH2, nothing from days or coded and the command deck, mirage arena, shotlock and command melding from BBS and finally from DDD, the flow motion system and maybe not the dream eaters themselves but a similar system of party management (keyblade apprentices maybe?).
I'd also like to see D-link system merged with the drive form system seeing as sora's friends are his 'power' I'd love to see get given a temporary outfit change like the drive forms and a change to his combat style and command deck like D-links, like a 'riku form' with more dark based and aerial attacks or an 'aqua form' with a more nimble magic feel... basically turning sora into megaman from the battle network series haha oh and on the PS3 of course.