In William Shakespeare's tragic play Macbeth, one could only assume that Macbeth would have become king of Scotland. The support one could use to justify this would be the fact that Macbeth was named the Thane of Cawdor (as seen in the prophecies) and the crown would have been the next title assumed by Macbeth (according to the prophecies).
The witches, in Act I, addressed Macbeth as Glamis, Thane of Cawdor, and king. Given that Macbeth had already possessed the title of Glamis and had been given the Thane of Cawdor title, even though it was unbeknownst to him, one could only assume that he would also (eventually) take on the title of king as well.
Therefore, if Macbeth would have allowed "chance" to crown him, based upon the prophecies, it is probable that he would have taken on the title of king as well.
Outside of that, Duncan had bestowed a new title on his son, Malcolm, in order to set up the line to the throne. Therefore, if Duncan would not have been murdered, Malcolm would have been the next in line for the crown. One would then have to assume that something would have to happen to Malcolm in order for Macbeth to take the throne.