Here's some things I would change about Tsubasa reservoir chronicles in my rewrite:
All characters are aged up. You CANNOT convince me that kurogane is younger than 30. And you're telling me Shaolran is FUCKING 14? No.
Shaolran has a brother now! Specifically Fujimoto from Kobato. (Kobato will not be in the story. It was a sus romance anyway)
Fujimoto is the one who takes over the archeologist crew when his dad dies, not Shaolran. He's older and has more experience. I'm thinking like 29 years at the start of the story.
The story starts with both Shaolran and Sakura confessing their love. No interruptions. (Why wasn't this in the og story? It would have been sadder???)
Kurogane is not an insane murderer that is addicted to killing. He's a political assassin and bodyguard for the princess. The princess just happens to be incredibly ruthless behind her cute facade.
Tomoyo banishes Kurogane not because he's killing everyone, but because having killed the rest of her political foes, he's the biggest threat to her. He's getting too suspicious of her.
The price Shaolran pays is his dad's dog tags. (Literally why would he pay with something that isn't his. Also why does yuuko need the equivalent of a handful of magical nuclear energy plants??? Think about it.)
I'll add more later, but this is what I got right off the top of my head!!