The subject of usurpation seems to have been very important to Shakespeare. It has a prominent role in many of his plays. Here are some examples of usurpers from eNotes study guides:
Duke Frederick—Duke Senior's brother; usurper of his dukedom. (In As You Like It)
Claudius—brother of dead King of Denmark; now King, and new husband of Queen Gertrude, Prince Hamlet’s mother.
Macbeth—Thane of Glamis, later King of Scotland.
Henry Bolingbroke (Bullingbrook), Duke of Hereford—John of Gaunt’s son and cousin to Richard II; later King Henry IV.
Antonio—Prospero’s brother, the usurping Duke of Milan, who helps Sebastian plot the death of his own brother, the King of Naples. (In The Tempest)
Richard, the duke of Gloucester, afterward King Richard III, the sinister and Machiavellian brother of King Edward IV.
In Shakespeare's King Lear, Edmund usurps his brother's title as heir to their father the Duke of Gloucester. It might be said that both Goneril and Regan are usurpers in that same play because they obtain their father's kingdom under false pretenses.
Iago in Shakespeare's Othello is something of a usurper. He undermines Cassio and secures his place as Othello's lieutenant.
There are other examples of usurpers and usurpations in Shakespeare's less famous plays. You can obtain a great deal of information for your essay by consulting the eNotes Summary of each of the plays referred to above.
There are many possible topics or themes you could use for your paper about William Shakespeare's Macbeth.
One possibility would be comparing Macbeth to Claudius in Hamlet. Both men have come to power by means of murder, and both end up being killed. Both men are ruthless, and although they are at times troubled by awareness of their own immorality, nonetheless act mainly out of ambition.
Another possible theme would be the supernatural in Macbeth. You could compare the role of ghosts and witches in Macbeth and Hamlet in terms of their role in decision-making and ethics.
Another possible theme would be a discussion of kingship. You could look at an assortment of kings in Shakespeare's plays -- Richard III, Theseus, Macbeth, Richard II, etc. -- and use that to investigate what Shakespeare considers a good or bad king.
A final possibility would be the topic of marriage. Is Lady Macbeth a good wife? We have several married pairs in Shakespeare's plays, and could use that to investigate what Shakespeare considers the proper role of a wife.