I was thinking about this stuff. I personally would love it if all super bosses were included but for the sake of simplicity the unimportant encounters would probably be excluded. Phantom can easily be skipped. One of Xion’s monster forms was seemingly based on Kurt Zisa, I think that makes the Zisa encounter somewhat important to the lore but it could probably be skipped without issue. Sephiroth’s KH1 appearance was already retconned even if that makes One-Winged Angel being in Chain of Memories awkward but whatever. Assuming Sephiroth shows up at all they’ll probably save him for the KH2 season. Ice Titan shows up in Recoded which means Jiminy Cricket wrote about him thus making the Gold Match canon in KH1...not that every detail of game canon really matters, the retelling will do its own thing, but the show might want to include Ice Titan because of this. But then we have something that cannot be skipped. The Unknown encounter.
Xemnas tasting Sora’s memories is a small but very important chapter of KH1 and a retelling of this fight has many interesting possibilities. Maybe they’ll reveal more information to the audience compared to what players got in Final Mix. Bigger hints or full reveals about Xemnas, Nobodies, replica program, Org 13, etc. The writers might not want to be coy with this sort of stuff in the retelling, they’ll need to hook watchers for the upcoming seasons based on CoM, Days, and KH2. Sora himself probably wouldn’t learn about this stuff but a show has ways of revealing information without the main character actually knowing, like a scene of Xemnas having a conversation with Xigbar just before he leaves to troll Sora in Hollow Bastion.