Johnny I think u need to chill out a bit, this is a manga/anime forum a hell of a lot of anime/manga is based off of impossible/unrealistic/borderline plagiarised concepts. Maybe it would help if u change ur name to pothead johnny or summin, I've heard that sometimes that kind of thing helps with the whole mean hat stuff 
When anime's golden age was beginning to roll it was funny what anime was "borrowing" from the US. Golgo 13 was james bond, Dog Soldier was Rambo, Lily Cat was aliens/the thing, etc.
This has turned around and Hollywood is looking to borrow from Anime.
The thing here is that people are looking to improve and put out good work. They do not want to end up writing the next Star Trek and endlessly gluing cliche's together with no continuity (well OK, with the paycheck that would bring, many of us would crank out dreck for money).
They want to get to the point that even if there stuff never goes anywhere it is still better than what they see on TV.
We are not looking at how to create a story, we are looking at how to create a good story and then get feed back and improve it.
Starting with someone else's ideas is a bad start but certainly not unworkable. If they are just an inspiration it is easier.
Think of it like cooking and everyone's stories are ingredients. The more well known the story the stronger the flavor of the ingredient. You can sneak some cinnamon into your lasagne but you cannot make it out of 30% cinnamon and have it considered "good".
Now fanfic and slash depend on using someone else's universe and characters, so that doesn't really count.