Julius Caeasar was a "man of the people" and yet had the Patrician lineage. The Senate thought they could control him and ultimately there was a successful plot to assassinate him which came from the senators who feared his potential power. Had Caesar not been assassinated, might things have worked out better?
That Caesar was ambitious is evident in his separation from his once friend and ally Pompey, a member of the first Triumvirate. He defeated Pompey and his allies in order to knit together the entire Roman empire. And, in order to accomplish his goal, Caesar felt he should control the government without dispute, so he increased his own power and lessened that of other political institutions in Rome.
Caesar was ambitious. Brutus feared his ambition. He admitted that he feared Caesar would crown himself king. Brutus stated that he loved Caesar, but that he loved Rome more. I do not believe Brutus would have killed a man he loved had he not feared that Caesar's ambition would result in making slaves of the people. It was Caesar's ambition that killed him.
I agree with #3. It was his ambition that was most dangerous in terms of the Senate, and this made him a character that they feared because of his power-hungry status. If he had not been assassinated, we have no reason to suspect that he would not have continued to gain more and more power. This would have been something that the Senate would not have tolerated indefinitely.
I don't know if it was his populist appeal as much as it was his ambition. Both had to be present in order for him to end up getting assassinated. A person who was that popular with the people, but who did not keep taking more power for himself and giving himself new titles might not have aroused the envy and fear of the Senate.
Caesar was the latest in a long line of dictators that had risen to power based on their popularity with the people of Rome. Yet many of these leaders, including the Gracchi, had ended up being assassinated too. What was different about Caesar was that he sought to make his position permanent. It is difficult to say whether things might have turned out better, Caesar wasn't the sort to be satisfied with what he had achieved, and it is unlikely that the new status quo would have lasted for long, especially given the social turmoil that accompanied the political disputes in Rome.