So basically you're writing a To What Extent paper (as opposed to a compare and contrast paper). You want to show to what extent did older cultures influence the writing of Romeo and Juliet.
First find some examples of this influence. If you can't, then you paper is- I looked, but I couldn't find it, sorry. If you can, outline them and explicate why/how they fit the question.
Show each example, show its origin in older culture, show where it fits into R&J.
Come to a conclusion about how this influence shows itself in R&J and what it has to say about Shakespeare's writing in general.
Each of these steps can be expanded until it's as long as you need it to be.
Don’t become a well-rounded person. Well rounded people are smooth and dull. Become a thoroughly spiky person. Grow spikes from every angle. Stick in their throats like a puffer fish.