Open theism, the limiting of God’s knowledge by human freedom which leaves our future open, could certainly explain the existence of human evil and suffering in the world, including the crucifixion of Jesus. (Greg Boyd recently debated William Lane Craig on the penal substitution theory. See the link below.)
The downside is clearly that it deprives the Christian of certainty that good will triumph over evil, as prophesied in the Book of Revelations and in the gospels. Jews believed the messiah would usher in the end times, when God will judge us and give us what we’re due. But if there are multiple possible futures that are left open even to God, that eschatological faith really does become just hope. Christianity would be more like Zoroastrianism in emphasizing the need for human participation to help God achieve the best outcome.
Moreover, the point of Jesus’s sacrifice would become less clear. Jesus would serve as a model or as a warning, but the lessons would be ambiguous. Are we supposed to believe God wanted to show what will happen to us if we stay on our present course of sin? Or was God apologizing for putting too much stock in freedom, underestimating the damages? Is the lesson instead that there is no God, because amoral empires can evidently ride roughshod over good people? That uncertainty undermines the call for moral action.
Finally, the talk of multiple theological futures puts the Christian on a slippery slope, since we can imagine God creating not just creatures capable of bringing about different futures, but multiple universes to test what happens. That leads to the thought of God creating every possible universe, which raises the question of why God would care about the outcome in, or about the inhabitants of any of them.