I will answer in more of a short term way than the first answer. I will confine mine to what happens to Winston.
It is implied in the book that Winston will be killed. I will have the book end with him not dying. Instead, I will end the book with him becoming an important member of the Thought Police and the Inner Party.
I think this would make sense because it would show how thoroughly the society could control its people when it really tries.
By the way, with regard to the first answer, I would encourage you to consider that the USSR fell apart to some extent because of the presence of the wealthy West that was a contrast to their world. Their people knew they were poor compared to the West and that helped make them want to rebel. There is no such outside contrast in 1984 so I'm not sure their system will fall apart.
I would probably end the story in much the same way as the Iron Curtain eventually crumbled in 1990. Instead of one single event causing the demise of Big Brother, I would envision a long series of failures by the government and a slow but sure realization by the people that it was time for a change. Oceania's power would decline, conditions would become even worse, and the disappearances of dissenters would grow until the people rose up to challenge Big Brother. The lower classes would have to unite with unhappy party members to create a mass too strong for even Big Brother to overcome. His images would be destroyed, microphones would be turned off, and the video broadcasts would end. Video screens would eventually broadcast entertainment of a pleasurable content suitable for all the people, with no hint of government interference--or Big Brother's control.