Boy, I needed this thread haha.
Firefly- I would go back and rewrite the pilot episode (or maybe the second episode) to really explain the characters' outbursts in Chinese. I mean, they told us a little bit about it in the box set featurette, but I want some awesome flashback or verbal confirmation or SOMETHING in the show that actually answers the question of why random Chinese is spoken.
Naruto- I would get rid of all the GODDAMN filler episodes in the anime. Stick with the freaking story!!
AC Brotherhood- I wouldn't even put in the Cristina memories. She just... bugs.
Twilight- I would kill Bella off. XD But, even more important than that, I would put an EPIC, FINAL BATTLE in the FREAKING FINAL BOOK. You don't string out four books and then just decide at the end, "Oh, we won't harm this mutant offspring that has the potential to kill everything in sight. Let's just leave!" YOU HAVE A FIGHT. AND SOME CHARACTERS DIE. Maybe, I would've actually enjoyed the series had it all actually led up to something.