I didn't really care about the difficulty. It's part of why the game was that much funner for me.
Things I'd change so I'd improve it:
Bring in more Riku and Naminé. Aside from them being two of my favorite characters, they're a great deal of the true plot of the game, more so than most of the Organization at this point in the series. What went on with Riku, Naminé and Axel is what really matters and could've impacted so much, and yet, they showed nothing.
Now, while rather in contrast to what I said, I'd kick Kairi out from most of the game. While she was truly kidnapped because Naminé asked Axel to help her meet her Somebody as told in the novels, it still doesn't remove the fact that Axel himself admitted to having done so for Roxas so we're left with it being gay and shoved in as it was before we knew about the novels, and in the long run it still served all but no purpose aside from an attempt to make Sora look less gay for Riku than he does. She's done nothing but not care about Riku, think about Sora, get so many people to gag over Riku lending her a Keyblade, and then she ended up being left behind again.
They could've given them so many other ways in which to get to the Castle, and Naminé would've better served the role of being the one to bring Sora and Riku together, having been with Riku for that year, and caring for Sora through that time as well. Even if Sora never would've learned the truth about her, she had a lot more right to bring the two friends together than Kairi did, when Kairi said Riku'd be fine on his own yet the instant she remembered Sora, waiting just wasn't good enough for her.
And let's not forget that Sora and Riku wanting to come back home means all but squat when they just met Kairi five minutes prior and they still agreed to stay behind and become the Darkness.
Sora was probably hungry or something so he wanted to come back.
She'd have had a lot more impact had she sent off the bottle and was there to wait for them when they came back. Had they not have met her in the Castle, there would've been a far greater meaning to them wanting to come back to Kairi - only whoopsy, Riku didn't really wanna, he only came back because Sora wanted him to. Lawl.
So yeah, total rewrite for her :\