I have a few (ok, a lot) but to give one example: I'd pick The Dark Knight. Id have the people on the ferries blow each other up (or at least blow the other ferry up) which would distract Batman long enough for Harvey Dent to finish his coin tossing with Commissioner Gordon and his family.
Then the Dark Knight Rises could have been about the aftermath of that, still featuring Bane and the League of Shadows but just from a slightly different angle, in that Batman wouldn't have been viewed as a murderer but rather a failure and Gotham would be completely without hope when Bane arrives.
Most Helpful Girl
I always wished that the Harry Potter series ended in the death of both Harry and Voldemort.
Most Helpful Guy
Not a movie, but i'd love to change the crappy ending of lost, the tv show. So disappointing.
End of watch, gyllenhaal get killed instead of m. pena.
The matrix ending was stupid, i would've change the whole 3rd movie.