1 Mercutio: Hey, here’s a good way to come up with stories that we can film!
2 Mercutio: We take the plot of some popular story and you write some minor variant.
3 Mercutio: Then we go back using my time machine software and register your copyright before the original work.
4 Shakespeare: That would be highly unethical.
4 Mercutio: Only if we don’t get rich off it!

Another option would be to write sequels to everything.

Just don't start writing webcomics, because that certainly won't make you rich.