[Act 5] What is the significance of Malcolm's ascension?
Finally, the mirror carried by the last king in the Show of Kings during the witches' Show of Kings in Act 4 Scene 1 represented the Stuart posterity stretching out "to th' crack of doom" (4.1.117); the the overthrow of Macbeth and the Malcolm, the rightful heir, meant not only a general purgation of the country's evil, but the establishment of a line of kings, one of whom sat in Shakespeare's audience hundreds of years later and was almost morbidly aware that the same evils continued to beset great men. The Show of Kings confirms the words of King James himself, who expressed the hope that he and his descendants would "rule over [Britain] to the end of the world," almost exactly matching the prediction of the witches.
In addition to the above answer, I would also say that for Shakespearean audiences, and for the king at the time, James I, it would demonstrate a return to normalcy - it would show that all had worked out properly in the end. The idea of regicide was considered unnatural at the time...a dismantling of God's natural order of things. So having Macbeth defeated and killed, and Malcolm (Duncan's son) crowned at Scone, would have confirmed for the original audience that all was right with the world.
I assume you mean Malcolm's ascension as rightful heir to the throne. In the end, the play has come full circle. Macbeth, who took the throne by blood and deceit, has been defeated. Malcolm's rightful rule has been restored. Malcolm assures everyone that peace will be restored to Scotland, and he rewards those who helped defeat Macbeth by giving them the title of Earl.