We will on occasion ask our players if they can work on continuing certain plotlines, but we generally don't dictate how or specifically what will happen, though if we need something specific, we could ask for it. The players asked can freely, without consequences for the future, decline such. Then, if the plan hinged on that and we cannot convince them, we simply have to change the plan.

As a GM, we like surprises. We don't share everything with the players, and we don't expect them to share everything with us. There's few things more enjoyable than an utterly unexpected, well-written plot twist from one or more player(s). Major plot is typically defined by the GM, but that doesn't mean the players cannot influence what happens. The story, while primarily directed by the GM, is a product of all the players as well.

If we were to have an RP prophecy that says "X slays the giant" we'd always try to ensure that happens, but it is the players' role to find ways to thwart the GM. Prophecies are often misunderstood, so if it turns out differently, then clearly it was misinterpreted.