Manifest destiny is commonly known as an idea that started in the 1840's by John L. O'Sullivan, advocating the expansion of the United States westward, through any means. That's not what I want to write about.
The word manifest means, "to be caught in the act". What a person truly is may be hidden, but from time to time, they are "caught in the act" of being their true self. I think also that we manifest when we are in movement or moved upon.
The word destiny means "determined". I believe in free will, but also that some things are determined. I lived my life and made day to day choices, but I believe that it was determined or my destiny to meet and marry my wife. Destiny is most easily looked at backwards and helps explain your life before a given event. To look forward to your destiny or to walk into it takes faith. Before I met my wife I felt destined to meet and marry someone like her. I believe I had words I heard from God that gave a glimpse of her. But until I met her and made her my wife, I was only looking forward to that destiny and believing for it.
So when I think of manifest destiny, I think of how do I right now, line up with what my destiny is? I think this is the essence of the prophetic life. As I heard Graham Cooke say, it's like we tie a rope to our prophecy that is out there in the future for us, and seek to bring our selves into becoming like the person that will have that prophecy fulfilled. This isn't some sort of self-fulfilling prophecy, but a wise answer to the question of, "what do I do between now and the time when my word is fulfilled?" A simple example is that if someone received a word that they would be a missionary, they would want to get some training.
So, for me, manifest destiny simply means watch how you are living today based on your future determined by God.