We've already seen that now with Locke. He wasn't destined to be the leader of the Others or fated to be. He went back in time and told Richard who he was and when he was born, etc... so he created his own "destiny" to be the leader of the Others, which means it wasn't destiny at all.
I have to say that really bugs me because of the predestined paradox that situation creates. The time traveling element right now is creating a lot of situations for such paradoxes and that has me worried. I'm willing to trust the writers to the end, but it is the one thing going on the show right now that concerns me.
They've always failed to disappoint on that count.
One little cycle that a someone at work theorized was that Desmond brings his son Charlie back to the Island and to the past. Somehow the son leaves with a Dharma couple and their other son and the man starts a butcher shop in England.
It's a crazy one but kinda neat.