"I will this night,
In several hands, in at his windows throw,
As if they came from several citizens,
Writings all tending to the great opinion
That Rome holds of his name, wherein obscurely
Caesar’s ambition shall be glanced at." I.ii.311-316
Brutus is Cassius target because he is so close to Caesar. If Cassius can turn Brutus, then the assassination will go well for them. Of course, we know that Brutus will turn. The end result is going to be that these two men are Caesar's chief opponents. Cassius the shady corrupter and the noble Brutus who's been convinced that this is the best way to go about it.
On the other hand, perhaps taking Caesar down would be a good thing. He has incredible power and authority. Should he go bad, it might be impossible for anyone to stop him. These opening scenes have introduced three of the most important characters, and set them up in how they will be interacting. How do you think this will develop? Will Cassius' plan work on Brutus easily, or will Brutus find out but still choose to side with him anyways? What could the turning point for Brutus be?