Did the conspirators want Brutus to take over as leader of Rome?
The conspirators did not want anyone to take over as Rome's leader because they felt that Rome's leadership had become corrupt. As a democratic nation, Rome was governed by officials elected by the people. Because of this checks and balances system, the idea was that no ruler would have absolute power.
But in Julius Caesar, the titular character has done just that. The people revere him as a god. We are told that Caesar has been offered the crown three times, which would solidify his position as permanent ruler of Rome. If he became the permanent and uncontested ruler of Rome, this would completely change the democracy into a monarchy, where the voices of the people would be silenced against the will of a king. Caesar's murder was less about finding a new ruler to take his place and more about restoring order.