John August spells out the keys to a romantic movie:
Characters we give a shit about. They don’t need to be likable, necessarily, but they need to be compelling. We need to be curious about what they’re going to do next.
A credible reason to keep them apart. This could be almost anything — war, prejudice, a sinking boat — but if we don’t buy it, you’re toast.
Actually, that’s basically translating to romance the basics of fiction: interesting characters and a conflict.
If you want to take it further, don’t miss out on the comments for additional ideas!