THE most about DDD is the unnecessary time travel. I'll never get over it.
We've had examples of time travel in the series before. Strange doors that traverse time and can affect the future. Why notttt stick with that? Why did Nomura draw up this weirdo "Ok so you gotta leave your body behind and then use clones to store your heart but eventually you have to go back in time anyway and you'll forget everything, but also you can't even change the past or future so the time travel is pretty irrelevant and yet it obviously DOES affect the timeline no matter how much we say it doesn't so there's that."
Just write up this cool plot where Xehanort/Xemnas/Ansem or whoever found or created this super cool fantasy doors of doom that he can control to move him backward and forward in time, just like the Timeless River doors? There can still be weird data scifi elements with that. Could work great with the whole book and data concept too. Sticks with the themes of doors. Sora and team could even revisit pst events from a new perspective! (and not in a CoD or Coded way where it's a rerun of past events) Sora and team trying to trace where the doors are to destroy/lock them? Maleficent trying to butt in and mess things up? What an awesome subplot --> main plot transition that would've been (just like Pooh's book is to the whole data worlds situation)
Why go through the trouble of bringing back your old selves, just make more clones, let them develop an identity, and go from there? I mean, I know Xion broke her leash multiple times but you still tried to use her in the end anyway soooo?
I know many people place BbS as the start of convoluted plot, but for me, it starts with DDD. Not necessarily because of how the plot presents itself (splitting the story between two people), but because of unnecessary story elements when they could've just used already existing ones. I can't think of anything in Birth by Sleep that was a brand new concept/idea except the idea of "Keyblade school", keyblade armor, Vanitas, and Old fart Xehanort as the true main villain. And none of those elements require deep thought, they're just supplementing things we already know (except Vanitas maybe?)
To this day I still struggle with Nomura's rules of time travel. I've accepted that KH is "soft magic", but it bothers me that he spends long cutscenes trying to explain how things work and then one game later those rules don't even matter or become even more complicated.