I agree that Romeo and Juliet drive the plot, but not solely. They have to have people helping them to continue their plan to being together. I have to say that two characters that contribute to driving the plot along are Friar Laurence and the Nurse. Friar Laurence is Romeo's father figure. He can't talk to his own father, so he always confides in the Friar when he needs advice. The Friar is the one who marries them secretly, the one who comes up with the elaborate plan to fake Juliet's death so she can get out of her marriage to Paris and run away with Romeo, and is the one who makes the potion for Juliet to fake her death. The Nurse, who is Juliet's mother-figure, is the one who does all the messagering for Juliet. She finds out who Romeo is, she seeks out Romeo and gives him messages for Juliet, etc. Romeo and Juilet are too young and impulsive to drive the plot along so well. If they had no help along the way, the plot would be all over the place. The lovers are anchored by the Friar and the Nurse who guide them on a more purposeful path to being together.
The obvious answer here would be the title characters. It would be Romeo and Juliet who exert the greatest impact on the dramatic plot. Without them, the plot doesn't go anywhere. It is their love, or their perception of being in love with one another, that drives the plot, heightens the dramatic tension and brings the play to its natural conclusion. Yet, I am going to posit a different answer and suggest that the parents influence the plot development of Romeo and Juliet. The parents are the ones who hold true to their feud and their antagonism. This is set up as early in the Prologue and it does not waver throughout the drama. For example, at the structural height of the plot, Juliet's parents pretty much disown her when she does not heed to their wishes and marry Parris. The parents' clinging to their ongoing feud fan the flames of the childrens' love, or their perception of it. Infact, one can argue that if the kids are seen as rebellious adolescents, then the parents are the ones who cause this with their hatred for the other. This would mean that it is the parents who greatly influence the plot development of the drama. When they cease to dislike one another, the play ends.