Just to add to the other great ideas--you could compare the role of influential women in the two works (Lady Macbeth and Julia). Both women cause the men in their lives to rethink their previous positions on various topics, and both do an excellent job of pretending to be someone they are not.
Similarly, the themes of psychological torture and not knowing whom to trust play significant roles in both works.
Great topic! You may want to go with the idea of illusion versus reality. Macbeth features witches, ghosts, and apparitions, all of which can arguably be factors and determinants for how the plot moves along and why certain characters do the things they do. 1984, things are hardly as they seem -- one day Oceania is at war Eastasia, next it's Eurasia. Mr. Charrington seems helpful, but is actually Thought Police.
I'd go with the idea of paranoia. Clearly Macbeth is driven by guilt and his ambition to become and remain king; however, those two things also drive him to paranoia. He wants Macduff dead, of course, because he understands he's a threat to his reign. Killing Macduff's wife and children and servants is simply paranoia. Likewise, many of Big Brother's actions and reactions are rooted in paranoia and the hunger for complete control. Both are out of control and unreasonable in their reactions (or over-reactions, much of the time) to the people and events which they confront. Interesting comparison!
Building on the previous post, you could do a contrast part about the effect that advanced technology has on the issue in 1984. Both had governments that wanted to rule totally, but it was much easier for Big Brother to be successful because of the enhanced ability to see and control everything that was going on.
Interesting topic there! You might want to tackle it from the angle of despotism and how each work considers tyranny but from different angles. Clearly Macbeth is a prime example of a despotic leader who aims to have complete control of his people and kingdom. He kills who he wants to and crushes all opposition until all opponents are dead or flee. Of course, the central and crucial difference between the works under consideration is that the tyranny of 1984 is of a much more totalitarian and unstoppable kind. Macbeth gets his comeuppance by the end of the play but Winston Smith, the underdog whom we are all rooting for, is broken utterly and conforms to the world of 1984.
You could write about the issues of government and the abuses of government and the results of those abuses. You could also write about the institutions of government and why they worked or failed to work properly for the benefit of the citizens.