A teacher once told me, of developing storylines, "You dance with the girl you brought to the party." If you start with a romantic comedy, and you've established that you're working that particular room, if you suddenly go Cassavetes in the last 20 minutes, it might feel right to you. But maybe only to you.
But I am now thinking of books and movies where a sudden breaking of the rules is liberating. Perhaps what they all have in common is boldness and declaration, so you know you're coloring outside the lines. No matter how ridiculous the landscape, if you trust the guide, the journey will be worthwhile. If you are led through the jungle and feel led then a romantic comedy with an unresolved ending might be okay.