Disappointed with the 13 vs 7 battle

[NobodyImportant]

Eh, they were in a position where they were damned if they did, damned if they didn't. They couldn't actually afford to make 7 playable characters, and if any of the fights were not playable you'd get a ton of people angry that they weren't able to battle so-and-so and it was just a cutscene between NPCs or something or other. I think it was a tough situation no matter what which would have lead to some people being disappointed so I think they instead focused on it being Sora, but you'd have new party member set-ups for each fights, some of them things fans have wanted for years themselves. Personally I think it's a fine enough compromise, my only wish is that maybe each battle had a significant finishing-scene with the relevant party members to make it feel like they contributed a bit more (i.e. we see Ven making the final blow against Vanitas after the battle) or something like that.

They didn't have to actually do so. If they had kept it to the 3 playable characters they already had in the game, maybe 3.5, it would've been more thematically appropriate.

Riku
Riku and Mickey were fighting together. Mickey gets forced into a card, Riku has to save him. They've been doing that all game, it would've fit fine. We did not need Sora rescuing two Keyblade Masters. Riku already has movesets in the game.

Aqua
It is an incredibly important fight emotionally and character development-wise for Aqua to fight against Terranort. Like Riku, you have her movesets already in the gamw, there's nothing new they would have to add in. It makes a ton more sense for Aqua to fight with NPC assists from Ven. Also she is another Keyblade Master, and this would have been a fitting conclusion to a redemption arc for her.

Roxas
He's the .5 for this. That fight would be been better if Roxas took over when Sora got knocked down. He's basically a clone of Sora's fighting moveset except he's got two keyblades. It makes sense that Sora does go to the rescue for this one done Lea and Kairi are newbies. But they could be briefly made Roxas a playable character too.

 

[Divine Past]

It's kind of funny if I am not missing anything there was nothing stopping Sora going to save Kairi right away if he loves her so much and having someone else fight Xehanort. Rather he decides to let me fight Xehanort and then try to save her. I remember Xion saying she feels Kairi was ok but why will I trust her if I am Sora.