I have given the subject tremendous thought and my own conclusion I feel comfortable giving is that opposition in the form of an evil person doesnt have to exist for the plan to work. The plan laid out in heaven was in place before Lucifer fell. It was a given that man would fall and a Savior would be needed. This evidence was already apparant before Lucifer fell. When lucifer fell and became Satan it says he knew not the mind of God. It meant that Satan tried to alter or change the plan to thwart God and take his power.
My firm belief is that our particular situation on this earth with the conditions as they exist is unique in the eternities. We have a different battle with a whole lot more at stake than what normally takes place amongst the Gods because of the great conspiracy that Satan was running in heaven and continues with here on earth.
My belief, and I feel very confident in it, is that myriads of worlds came and went, myriads of generations if Gods came to pass without much if a hiccup and then came Lucifer, a spirit of high power and authority in heaven second only to Christ, who used strategy and conspiracy in the courts of heaven to overthrow the kingdom. He was found out, his secrets God countered with his own, which then laid the foundation of our particular earth where the great test would take place and victor takes all in the end. So much being at stake, God took his best most valiant spirits and sent them here to earth to ensure the kingdom of the devil would be destroyed. Satan has all his chips on the table now playing out on this particular earth.
In one sense, we are living out a scenerio here on earth that has perhaps never come about in the eternities. God allows Satan because, after all, Satan did establish his kingdom on the principle of free will. I would bet that because of the situation here, our experience is unique in the history of eternity. God will destroy Satan and all his works in the end. What will that mean? It probably means that another situation like this will possibly never arise again because all of the secret plans of Satan will be destroyed and the children of the Gods will no longer learn evilness in heaven to ever again make war with God. To me this means that new provisions will be put in place that will prevent the scenerio of Lucifer ever happening all over again in eternity. God doesnt have to have evilness to fulfill his plans. But, since we have it now, God uses it for his own good to better preoare his elect to destroy Satan and all his works.