Worse case scenario, they could just emulate it. Game doesn't look that bad in HD:
Obviously 2D elements like the HUD & menu would have to be updated, but that's about it.
That said, if they
were to do a full-on remake:
1. Don't have Story & Mission modes share every mission. This is the biggest fundamental problem IMO. Every multiplayer mission has to first be cleared in story mode. This means that
A. Mission Mode has no exclusive/unique content &
B. Story Mode is padded longer than it should because it had to contain every mission in the game.
This was ridiculously shortsighted planning that made BOTH modes suffer. Story needed to be leaner & to the point. Multiplayer needed a wealth of new content for players to experience. Story goes on for what feels like forever & when it's over, there's nothing new to actually do in Mission.
What makes this even crazier is that Nomura said they originally planned a full 358 missions & a Pinocchio world but had to cut those because Story Mode was getting too big. Well then why not make all that Mission exclusive?! A whole new Disney world would be a hell of an incentive for players to keep playing.
Honestly, if anything, Days should've been structured like Crisis Core: chapter based main campaign with 300+ side missions that fill in tertiary plot details.
2. More Disney Bosses. Really, only Pete & Oogie's brats? Everybody jumps on KH3's case when Days was so much worse about it. Put in Hades, Add Hook, have the Queen of Hearts grow big from an Eat Me cookie. & most of all, have Gaston in it, make it that the reason Beast even listens to Xaldin to begin with is because they saved his life from Gaston's attack. If Pinocchio comes back, throw in Honest John or Monstro.
3. Do more with Disney characters/worlds. Peter Pan doesn't even appear in his own world for Pete's sake! The whole "org members keeping out of sight for continuity reasons" doesn't hold water when half the worlds in this game weren't even in KH2. What would it matter if Roxas & Pan met? Or him & Beast when Beast already knows of Xaldin? In fact, why did we never see Xaldin & Beast meet on-screen? Literally the main reason that world was included to begin with & they didn't show it!
Have this whole arc where Roxas finds Jafar's lamp in the Cave of Wonders & tries to wish for Axel's return (with this happening when Axel was persumed dead). Xigbar tries to manipulate Roxas into making wishes for his goals & Jafar tries to find ways to screw them both over & get wished free.
4. Rebalance characters. Magic heavy characters are screwed when they run out of spells, attack heavy characters don't have that problem. Also, how is it that Goofy & Vexen both have average defense when they're the ones with shields?
5. Make other characters besides Roxas playable in Story. It's weird that the game doesn't actually utilize its biggest gameplay hook in the story mode & only has you being 1 out of 19 playable characters. Let us be Xemnas & learn more about his schemes 1st hand or something.
6. Add Christmas Town. Because I have a weird desire to see Xigbar in an elf costume wielding candy cane arrowguns. & now you do too.
7. Unlockable alt costumes for everyone. By this point, we've seen every playable character except Demyx & Luxord in different duds & Nomura's probably has ideas for those 2 as well.
Xenmas-
Dalmatian er, zebra suit
Xigbar-RG guard
Xaldin-RG guard
Vexen-lab coat
Lexaeus-RG guard
Zexion-lab coat
Saix-KH3 ending clothes
Axel-KH3 ending clothes
Demyx-???
Luxord-Limo driver
Marluxia-Daybreak clothes
Larxene-Daybreak clothes
Roxas-Twilight Town clothes
Xion-KH3 ending clothes
Sora-DDD
Riku-KH1
Goofy-Castle guard
Donald-Castle mage
Mickey-KH2
8. Kairi, TAV, Ansem SoD, & YX as playable. Because why not? More alts:
Kairi-KH2/KH3 clothes, pending on which is default
Terra-Armor
Aqua-Armor
Ven-Armor
Ansem SoD-Org coat (this costume used for another phase of the Riku & Roxas duel)
YX-Destiny Islands clothes
9. Better final battle between Roxas & Riku. Wow, Riku was a pushover. I get that Xion was meant to be the spiritual final boss of the game, but there still could've been more done with Riku. That was the fight the series had been hyping since the 1st game, it needed a bigger payoff & not be a super easy fight that felt like an afterthought following Xion.
10. Both splitscreen & online. I'm a stickler for local multiplayer. Square may want a Switch version for local handheld play like the original version too.