An answer for either or both would be preferable. I am uncertain about whether he represents the powerful nobles looking to the gain the throne as seen by Caesar's ambition or Queen Elizabeth as the state descends in chaos following Caesar's death representing the threat of civil war in England following the Queen's death.
Historically speaking, I doubt Caesar would represent nobility. This is because nobility was something he very much looked down upon, as encapsulated by the distaste between him and the Roman Senate, which was filled with the aristocracy. If anything, I would say Brutus would represent nobility. It sounds more plausible that Caesar would represent Queen Elizabeth, but still they aren't really comparable since Caesar was a dictator and Queen Elizabeth was a monarch.
I suppose similarities can be drawn with the shared anxiety surrounding Caesar's and Queen Elizabeth's rule. In the end I don't think there's any decisive answer that could be drawn to answer what Caesar's meant to represent - Shakespeare could've just wanted to recount a Roman tragedy like he has done for Antony and Cleopatra. Regardless, you can choose to support either viewpoint, as long as you support it with content from the play.