I wish I could contribute to this thread but I fear melody is beyond my understanding. It is a complete mystery, an ineffable and magical thing which seems to defy all systems. A complete tyro can set the world singing with something born of poking about for a while with a couple of fingers on an instrument. Can we hope to systematically increase our likelihood of producing strong, moving melody ? I'm not too sure about that either. Those composers who have produced many powerful melodies don't talk about it, and probably don't know themselves how it happens. "It just came to me", is usually all that is said.
Put it this way, on a pragmatic and personal level, I could say with absolute certainty that I could play for half an hour and create reasonably good piano music. No way could I be certain a melody within it will be in any way memorable. Sometimes they even spring into the mind at odd times and places. One of mine I consider very strong burst on me while I was standing at a city intersection waiting to cross the road on my way to work. Just poof ! There it was.
I think we can say that a strong melody is firstly simple and secondly speaks directly to the heart and not to the mind. It is easy to see formal properties, symmetry, certain harmonies, balance of phrasing and all the rest of it in retrospect. But it is also easy to write thousands of melodies which have all these intellectual properties and say absolutely nothing.
Topic: Project Melody Writing