These are my suggestions for tiny changes that would have saved terrible movies - no takers this time around:
#1 needs no explanation, I hope.
#2 - Nah, that wouldn't have saved that trainwreck either, now I think about it...
#3 Yes, I know, War of the Worlds is considered a classic by many, but I really don't like the Martian War Machines not having their three legs...
Last but not least - Vader's "Birth" in the 3rd prequel kinda reminded me of something...
Same idea - younger Frankenstein: