What "vision of empire?" Even by the end of his life it's very clear that Caesar didn't really have much of an idea of what to do about the state, and the direction in which the state was beginning to move under Caesar was, although still broadly similar, very much different in a lot of important ways from Octavian's later brainchild. There's absolutely no reason to believe that Caesar had any sort of grand plan for the state, although he undoubtedly understood that it was sick--that was something that Cicero makes quite clear everyone understood. Even Shackleton Bailey, who was no fan of Caesar's, admitted that at best Caesar had a sort of fuzzy idea that something had to be done, but only by the end of his life and only in a very tentative sort of way. It's still hotly debated where Caesar was planning to go with, and following, his dictatorship--what was the deal with the rejection of the crown (personally I think that it may well have been Antony's idea and Caesar might have had little to do with that particular episode) and all that? I think Gelzer's argument, which most people would agree with in some form, that the entire purpose of Caesar's life was to outshine everyone in dignitas is spot-on--even during his dictatorship many of his actions seem to indicate that he thought of himself as merely the most authoritative man in the state, the holder of the greatest dignitas and auctoritas, but not necessarily the state itself, and I definitely think that it's true that the major driving force throughout Caesar's career was his desire to simply be better than everybody. That is, to advance his own dignitas. This is a topic that is heavily debated, and to say simply that Caesar had some plan to overthrow the state is at best fallacious. Nor is it really right to call Cicero a Republican, at least not as something opposed to Caesar (not yet at least)--Cicero defended the constitution to the best of his ability (though he famously violated it in 63), but Cicero agreed with Caesar on a number of points and in 60 was hardly a friend to the nobiles. His successful opposition to Rullus' agrarian bill in 63, for example, was not founded on a belief that the ager publicus must not be distributed, as many in Cato's circle thought, but that the method by which Rullus proposed to distribute it (the decemvirate, a board of ten--the same law that Caesar would pass in 59) was unconstitutional and dangerous, since it granted enormous powers to what he called ten "tyrants"--in fact, Cicero in his speeches de lege agraria presented himself as a populist supporter of Pompey's!

In 60 Cicero was in quite an authoritative, but extremely delicate situation. His dealings with the Catilinarians had gotten him hailed pater patriae, and he had an enormous amount of auctoritas whether you agreed with him or not. But his dealings with the Catilinarians were also very divisive--few would argue that the Catilinarians had not deserved to be punished, but Cicero had violated the Twelve Tables and executed them without trial, without the option of going into exile (which defendants often did during their trials, c.f. Milo). One of the Catilinarians, Publius Lentullus, was even a serving praetor! Besides this, Catiline had had surprisingly widespread support, although Cicero claimed that the basically the entire city condemned him at one point or another. Cicero's decision to hand his proconsular command to Antonius prevented him from receiving promagisterial immunity, and there were quite a number of people who were ready to prosecute him--already at the end of his consulship Cicero was prevented from delivering the customary farewell address that consuls gave by the tribune Nepos, who interposed his tribunate veto on the grounds that nobody should be allowed to address the people who had executed Roman citizens without trial. As the OCD says: "Though it was unlikely that he would escape prosecution, Cicero refused overtures from Caesar, which might have saved him at the price of his political independence." Support for Caesar during his consulship offered Cicero both the opportunity to remain safe from prosecution and the chance to see a number of laws that he supported passed--but Cicero considered the way in which Caesar hoped to secure the consulship and his intention to use his political partners to strongarm the opposition a violation of constitutional principles, and would have none of it. In the end, Caesar's suggestion that Cicero would not remain unprosecuted forever was entirely true--in 58 Pompey turned Publius Clodius, whom Caesar had transferred to the plebeian order to enable his election to the tribunate and whom Cicero had antagonized after the Bona Dea scandal, loose on Cicero, resulting in his exile in 58

To further the question: are Robert Harris' books "Imperium" and "Lustrum" historically accurate or does he add things/hypothesize a lot?