I would really like to see Cloud and Sephiroth get their showdown, Zack again since he could tie in with Crisis Core/FF7 Part 2 or 3 and we have no idea what happened to him between BBS and KH4 other than the black feather we saw, Jack Garland just because I'd love to see his energy mix with Sora, and maybe a villain, even if he's just a background character. As your post implies, maybe we see Kefka at a circus?
Other than that, I'm not really sure that they'd go with anyone else below 6 as it felt very random to have Setzer in KH2...so possibly Lightning finally in a Lightning Returns capacity, and possibly Noctis as an old man. He could be Yozora's father even, as a tie in to how FF15 ended.
Auron could also return, maybe we can see Braska and Jecht as well. I doubt they'll ever acknowledge that Tidus, Wakka, and Selphie were on DI ever again but Jecht could make smal talk with Sora about his son. Of course most all of this is based on the parallels between death and KH4...