The dagger foreshadows Macbeth's mental instability. He begins to lose his mind once he decides to murder Duncan. Knowing the murder to be wrong, Macbeth still moves forward with the heinous act. The appearance of the dagger solidifies Macbeth's future dark path (one filled with murder and madness). The dagger represents Macbeth's first step towards a life driven by ambition (his hamartia).
Although not explicitly stated, one could assume that Banquo's children face dire circumstances as they are forced to live without their father. Outside of this, the play does not offer any direct explanation of another character who faces dire consequences because of Banquo's ghost.
William Shakespeare offers readers multiple instances of foreshadowing which cue readers in to what is to come for the characters of Macbeth.
The Dagger illustrates both Macbeth's murder of Duncan and his impending mental instability (signaled by his hallucinations). No one but Macbeth sees the dagger, therefore it only affects Macbeth and his own future.
Banquo's ghost directly impacts both Macbeth and his wife, Lady Macbeth. The appearance of the ghost at Macbeth's coronation dinner shows Macbeth to be imbalanced. As he looks at the dinner table, Macbeth notices that no chairs are open for him to take. His guests question his mental stability, noting that a chair does sit empty. Unfortunately, Macbeth sees Banquo (or his ghost) sitting in his chair. Lady Macbeth must come to his rescue to defend her husband's mental state. Here, Banquop's ghost opens up Macbeth to the criticism of his guests.
The Apparitions' prophecy lead Macbeth down the path to his destruction. If Macbeth would not have misinterpreted the prophecies, Macbeth would have been able to be far more careful with his future. By misinterpreting the prophecies, Macbeth insures his tragic fate.
Thanks for your answer how exactly does the dagger affect macbeth's future? Also does banquo's ghost have dire consequences for any other character than macbeth?