I don't know what Propp's morphology is (sorry), so I'm going with Frye's archetypes (see below).
Romeo and Juliet is only border-line tragedy, as far as I'm concerned. I would say that it's half tragedy, one fourth romance, and one fourth comedy.
Or, you can break it down by act: Act I and II are romance and comedy, and Acts III, IV, and V are tragedy. It's almost too plays in one.
And, you can break it down by character: Mercutio and the Nurse are in a comedy/romance, and Romeo and Juliet are in a tragedy.
According to Frye's archetypal characters, you can say that the play is a romance and that Romeo and Juliet are on a quest for love, though I wouldn't say anyone is ideal.
So, the Hero (Romeo) / Heronie (Juliet) are aided by Helpers on the Quest (Friar Lawrence / Nurse).
Enemies of the Quest are not your typical giants, orgres, or evil madmen. It is Tybalt, obviously. But also Fate ("star-crossed lovers") and the families' Hate, which could be the same thing, really.
Sprits of nature are the lark and the nightingale from Romeo and Juliet's bedroom scene and all of the light, dark, night, and day imagery. Enough already!
Realistic Counter Companion who calls attention to fear, the jester who deflates romantic ideals is Mercutio, obviously. His Queen Mab speech is an attempt to derail or counter the love quest.