Alright, well I figured “With or Without You” by U2 would be great for the final act. The lyrics are very simple but do seem relevant (to me anyhow):
“On a bed of nails she makes me wait and I wait without you.” (Romeo anxiously waiting to hear back from the Friar about Juliet. I suppose this is closer to the end of the fourth act but I think its still relevant).
“My body bruised, she’s got me with nothing to win and nothing left to lose.” (After a fight with Paris he discovers Juliet seemingly dead. This one leads nicely into the next line…)
“I cant live with or without you.” (They can’t be together because of the feud between the families and they can’t bear to be apart [one of them dead] thus committing suicide).