In Macbeth, who does Macbet wish dead? What part of the prophecy would this change and why?

Macbeth organises the assassination of Banquo and Fleance
in order that he will retain the throne on Duncan's death. However, he has more than one
obstacle in his way.

Macbeth is perturbed when Malcolm,
King Duncan's son, is proclaimed Prince of Cumberland in Act I scene iv
-

The Prince
of Cumberland! That is a step
On which I must fall down, or else o'erleap,

For in my way it
lies.

The title of Prince of
Cumberland is akin to the current UK title of Prince of Wales, that is, official heir to
the throne. Of course Macbeth does not wish a named heir to stand in his way, so wishes
Malcolm dead. As Malcolm flees, this plan is temporarily
suspended.

He is, however, most wary of his great friend
Banquo, as the prophesies delivered by the witches  in Act I Scene iii suggest that he
is more of a threat to Macbeth -

Thou shalt get
kings, though thou be
none.

 So all hail, Macbeth
and Banquo!

However, as well as
Banquo's death, Macbeth requires Banquo's progeny to be killed also. This is Macbeth's
third target; Banquo's son, Fleance. Their deaths would render the prophesy
untrue.

The audience would probably know that it was
Banquo's line which did in fact take the throne, so would know that Macbeth's wicked
plans would be thwarted somehow.