Take any new Hollywood movie. There's a good chance it's a sequel, prequel, remake, or based on a television show or blockbuster novel. Few movies are made based on original material.
Take any new Broadway show and there's a good chance it's a revival, extravagant musical, or one-person show. Few new Broadway shows are straight-forward, non-musical plays.
Next, look closely and most have little freshness infused. Hollywood and Broadway are notorious for producing "safe and bankable" shows. Unfortunately, this leads to mediocrity and flops.
They know it; it's part of their business models: produce sufficient shows and one is bound to become successful enough to foot the bill for the rest.
Fortunately, indies, off-Broadway, and myriad small theaters actively seek freshness, which creates opportunities for scriptwriters ... if they can provide that freshness. Therein lies the challenge.
Many schools of thought declare there are only a limited number of plot lines, and all stories are variations of those archetypes. Some say a handful; others list as many as 20.
First, determine your theme. Writers want to make statements, increase awareness, or present possibilities for modern dilemmas. For consistency, it is imperative that the writer keep this theme in mind always. A common flaw in writing occurs when a script starts wandering among multiple plots, themes, or unrelated situations.
Themes are generally one or two words, such as jealousy, revenge, self discovery, or societal exposè. The plot, or storyline, then becomes the vehicle that depicts the theme.
For instance, jealousy may be presented through a love triangle plot. But those three characters may be in the near or distant future, past, or in the present, and may be in the city, country, a foreign land, or even non-human.
With so many options, what do you do? Here is apoint of great decision. Here is where you can take the safe and familiar path, likely based on what you've seen before.
This is also the point where you can, as Bach frequently did with his music, create variation upon variation until you reach the one that not only depicts your theme with clarity and intensity, but also presents opportunities for you to really use your creative tools.
Scriptwriting should not be a footrace to see how quickly you get to "The End" or "Curtain" by merely writing the first things that come to your mind.