Most of the stuff I can think of is probably being changed in the remake, but I can list some stuff anyways I guess.
Update graphics
Fix the translation
Make the Golden Saucer mini-games not terrible (most of them are bad)
Skippable cutscenes
Maybe make Enemy Skill not so crazy good
Speed up/cut some of the dumb optional stuff like Chocobo breeding, cpr, the whole lingerie gathering, etc
Make a better cast (only care for Yuffie, Cid and Barret)
More variety to Limit Breaks
Unique materia maybe so everybody isnt basically the same)
Cant remember if it has it or not, but skippable summon animations would be nice too because Knights of the Round takes so long you can make a 5 course dinner
There's like nothing to the battle system. Even by 1997 standards there's no depth whatsoever there. Probably the most shallow feeling RPG I've ever played in that department. I'm not really sure exactly how to fix this with one change, and I don't really wanna spam with changes.
Make the characters more distinctive than limits, starting stats, and weapon range I think would be the biggest single improvement to make there.
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Make KotR and the other few late-game materia accessible only after the final boss and add in meaningful and difficult post-game areas with tougher enemies and bosses. A lot of the super strong materia trivialized the game and it would be fun to see them actually used in high level enemy battles.
Remove game-breaking early on. Prevent abuse of enemy skill.
In its time it makes sense that there isn't any post-game content, but the opportunity to remake the game and add said content is very appealing in modern gaming. I'd love to see where that would lead.
Improve the translation and clarity of the plot
Graphical update for the models
Add more mechanical distinction between party members
Update the music quality without "remixing" it
Having "secret" characters is something I actually miss in RPGs. I thought Yuffie and Vincent still added enough to the story (Vincent especially) without being 100% integral if you happened to miss recruiting them. This was so much improved over Mog/Umaro/Gogo from FFVI.
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