Everyone who writes does so for their own reasons. I got into it because I got tired of coming to the end of an article or story and thinking, "I could do better than that." (OK, to be honest, my friends got tired of me saying that and told me to put up or shut up.) I don't write much any more, but I still have that reaction.
For example, Aliens 3 was on the other day. It sucked. The first two films were great, but the third just felt tired. You know what would have been a better plot? Ripley discovers that there's a third stage in the aliens' lifecycle, one that's intelligent and civilized. They drop eggs on a new planet, wait for stages one and two to wipe out everything dangerous, then poof, out comes Stage 3 to give you paved roads and quantum physics and what-not. The tension in the film would be whether or not Ripley could leave her "unfortunate" early experiences behind and connect with the third stage aliens or not, which would have been very topical for an American film as communism came to an end in Europe.
And what about Battlestar Galactica? Lots of great writing in the first two seasons, but come on, the ending? They walk away from hot coffee and decent medical care to breed with Australopithecines and make us? Bah. Bah, I say, and double bah. You know what would have really rocked? If they'd found the two-thousand-year-old hulk of the Galactica in orbit around Kobold, and spent the next two seasons figuring out how it---how they---have traveled back in time, and whether the past was written in stone, or if they could change it somehow. If nothing else, it would have been a much better use of the black hole that the evil Cylons' base was parked in front of at the end of the series...
What makes this more than just sour grapes is the prospect of actually being able to make "my" versions of these movies. Digital animation continues to go from strength to strength. Ten years from now, studios will be able to reverse engineer digital actors from old footage; not long after that, I expect technology to reach the point where dedicated amateurs can sample and remix film, just as they now do with music.
So, what would you re-write and re-make? A Terminator 3 in which Arnie plays the (human) scientist chosen to carry on with Miles Dyson's work, and John Connor tries to stop him? What would you do first?