Which character poses a threat to Macbeth but escapes an assassination attempt?

There also is another character who poses a threat to Macbeth who escapes assassination and that is Banquo's son, Fleance. After the surrender of Norway in Act One, when Banquo and Macbeth are returning from battle, they encounter the witches who give each man future predictions.  They prophesy that Macbeth will become Thane of Cawdor and then King of Scotland. For Banquo they say, "Thou shalt get kings, though thou be none" (I. iii.). 

The word "get" here is short for the word "beget," which means "to generate offspring."  Banquo's son and the future males of his bloodline will be Kings of Scotland.  This is in fact true because the real life James I, King of England and Scotland, for whom Shakespeare wrote the play, was descended from Banquo and Fleance.

After Duncan bestows the thaneship of Cawdor of Macbeth because the former Cawdor was a traitor who has been executed, and after Macbeth murders Duncan to ascend to the throne, he is upset that he has no heir and that Banquo's children will inherit the title.  To Macbeth, this means he has damned himself for someone else's family bloodline.

He says: "If 't be so,
For Banquo's issue have I filed my mind;
For them the gracious Duncan have I murder'd;
Put rancours in the vessel of my peace
Only for them; and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!" (III. i).

In order to right this wrong, Macbeth directs the murderers to not only kill Banquo, but also Banquo's only son.  During the attack in the forest however, Fleance escapes and his dying father tells him, "Fly, good Fleance, fly, fly, fly!
Thou mayst revenge" (III. iii). 

Even though Fleance is but a young boy, he is still alive and therefore can reestablish his family bloodline.  The reader is unsure of whether Fleance knows that the men who have murdered his father have been hired by Macbeth. But the upshot is the same.  This is confirmed by the witches when Macbeth goes to see them in Act Four and they show him a line of eight kings all descended from Banquo.