I'm guessing your question is supposed to slot into a specific chronological moment in the play; after the Macbeths have murdered Duncan. And the answer then is - well, the sons of the crowned king are the next logical targets: Malcolm and Donalbain, but they run off to England and Ireland respectively.
When Macbeth is on the throne, he immediately thinks of the person who the witches said would prove father to kings: Banquo. And it's Banquo who Macbeth orders to have killed next.
Except that "the Macbeths" don't kill anyone. Don't forget that Lady Macbeth says that she would have killed Duncan, but she didn't:
Had he not resembled
My father as he slept, I had done't.
It's Macbeth who actually does the deed. And when it comes to killing off their next victim, it's Macbeth on his own. When Lady M asks him who he wants to kill, he doesn't tell her:
Be innocent of the knowledge, dearest chuck,
Till thou applaud the deed.
The murders which follow that of Duncan: the grooms, Banquo, Macduff's wife and family... and many nameless others, are all Macbeth acting by himself. One of the crucial points about Duncan's murder is that it also kills the Macbeths' marriage.