Seeing as you've taken your inspiration from the prologue of the play, you've gotten a key factor in your argument already. The play starts by informing the audience what will happen. The "star-crossed lovers take their life." As such, Romeo and Juliet's fates are determined both figuratively and literally. It's a wonder that the audiences even bothered to stay for the show!
That being said, one other element that you might look at is the fact that Romeo and Juliet's relationships are pre-determined by the feud between their families and parents. They only find out about each other through chance and once they do, their relationship is very dangerous. Considering the fact that their families don't even know about their relationship, it's pretty significant what happens to them.
Symbolically, we can turn to both Romeo's and Juliet's desperate measures to maintain their love for each other. Romeo is banished, which comes about through chance, but also through the feud between the Capulets and the Montagues. Juliet is supposed to marry Paris, which is illustrative of her "locked fate," but she takes action to prevent that. She was about to commit suicide when Friar Lawrence offers her a potion that will make it look as though she has died.
O, bid me leap, rather than marry Paris,
From off the battlements of any tower,
Or walk in thievish ways, or bid me lurk
Where serpents are. Chain me with roaring bears,
Or hide me nightly in a charnel house,
O'ercovered quite with dead men's rattling bones,
With reeky shanks and yellow skulls.
Or bid me go into a new-made grave
And hide me with a dead man in his shroud
And I will do it without fear or doubt,
To live an unstained wife to my sweet love.
Here, Juliet is very much committed to solving the problem with the potion. However, her language and circumstance illustrate and foreshadow that this will not go as planned. She's locked into the fate of death, both by the prologue and by her naive and rash committment to the actions she will take.