There are LOADS for you to choose from. For me, my personal favourites are as follows:
Comment on the theme of appearance vs. reality.
Discuss the development of the character of Macbeth, in particular focusing on his presentation in the final scenes of the play.
Compare and contrast Banquo and Macbeth and their responses to the prophecies of the witches.
Analyse the marriage of Lady Macbeth and Macbeth.
In addition to these, you can also examine any one of Macbeth's soliloquies and analyse how it relates to the play as a whole and the development of his character.