What do you think would make Days a better game? Personally I have a rather extensive list in the spoiler below but tell me your own thoughts.
Off the top of my head and in no particular order...
Do more with the organization members. Maybe even address their pasts. Go into Marluxia's motivations for betrayal more, explain the method behind Vexan's madness. Show the other sides of Zexion, Demyx, Luxord. The others are harder to flesh out but it's an avenue worth pursuing. If possible even go into detail regarding Xemnas.
Make the combat more varied, less drawn out. So many of the fights in the game were repetitive.
Ditch the limited magic system.
Make the Organization members really powerful in mission mode. Controlling them in such a weakened state was a big letdown.
Make the story in each world more substantial. For most of them it's as if a 1-2 day story stretches their entire runtime. Add more worlds. Of course this is no quick fix, but it's still an issue.
Explain how they tricked Xion and Roxas into thinking the others were heartless.
Don't make Xion tell Roxas to free Kingdom Hearts if she's just gonna get Riku to stop him.
Don't make The Ruler of the Sky a chase battle.
Make all of the missions things that you'd actually expect The Organization to do. Recon was a good idea, but it was executed poorly. There's no reason for the Organization to make Roxas collect Sigils. Shadow Blobs are a contrivance. Etc etc.
Ditch the Stealth missions.
Make the fight with Riku more, well, climactic. It was a big letdown after the fight with Xion. And it didn't seem much like a fight between Keyblade Wielders.
Don't section off parts of a world then have Axel try to explain it in-character. The organization doesn't go around putting barriers everywhere.
Don't give the organization foreknowledge of the heartless you'll fight on missions. That's an unrealistic amount of recon, especially when Heartness come and go like the wind. It makes little sense for you to be searching for a specific Heartless if it's resting in the Corridors of Darkness until you go after it.
Don't make the corridor of darkness a stationary object. Let the characters use it like they normally would.
Fix the plothole with Riku by having him actually fight Roxas once before the end of the game.
At least tenuously tie Roxas' Samurai nobodies to him. It'd be great if he could actually control some.
Ditch the Dustflier.