The ending: order restored?
The poison--that which was rotten in the state of Denmark--has been purged by the deaths of all those affected, and an ambitious, powerful figure is now likely to succeed to the throne.
We learn little of Fortinbras, since he is on stage only briefly; but what we do know fits him to some degree into the type of the ideal prince sketched by Machiavelli.
The endings of many of Shakespeare's tragedies are more "open" than one might expect: the figures left in authority do not always inspire confidence. In Macbeth, Malcolm is an equivocal figure, whose children do not rule in Scotland; the Triumvirate in Julius Caesar is evidently unstable, and it is not altogether clear whether Edgar or Albany takes over after the death of Lear*.
Footnotes
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The last word
In the Quarto version of Lear, published in 1608, Albany speaks the last lines of the play; in the Folio (1623), the lines are given to Edgar.