What does the detainment of Friar John due to his possible contact with a deadly plague suggest in the text? Could this act as another instance of symbolism?
Friar John's detainment as a result of his possible contact with a terrible disease seems to be just one more way that fate is working against Romeo and Juliet. Friar Lawrence's plan for Juliet to fake her death and thereby escape her marriage to the County Paris is relatively sound; it just depends on this one thing: Romeo receiving the letter that acquaints him with the details of the plan. However, as early as the Prologue, the Chorus says that Romeo and Juliet are "star-crossed lovers" who are "misadventured"; thus, we know that fate will work against them in the play, and an unexpected and previously unheard of "pestilence" is just another example of a terribly unlucky, unpredictable thing that could happen.
Just as no one could have predicted that Tybalt would challenge Romeo just hours after he married Tybalt's cousin, or that Tybalt would kill Mercutio when Romeo came between them (an action that should have protected Mercutio rather than led to his death), or that Romeo would slay Tybalt, some random disease is just an unfortunate and arbitrary event. Therefore, Friar John's quarantine seems like just one more symbol that fate will intervene to prevent Romeo and Juliet from being together, at least in this life.