The Caracella bust as you mention was probably created to look so aggressive as the potential for an attempted coup was ever present.
Another thing to consider is that all these busts were probably painted. Which in some instances could change the whole effect of the work.
The above is based on pigment traces on the prima porta Augustus.
I think Romans in both the Republican and Imperial periods had an obvious love of realism, a hellenistic influence, but then I also think that idealism played a part in both periods as well. I think the Roman tendency to emphasise facial quirks, age lines and like features are often deemed "realistic" in our own time, as our idealism, and idea of beauty and virtue is not the same as the Romans. A Roman republican Patrician wanted to appear older, wiser, stern, intelligent and militarily capable, a paragon of experience, strength and wisdom. Augustus on the other hand wanted to appear God like, as did many of his successors. Vespasian possibly wanted to hark back to those republican ideals, and emphasise solid, wise rulership by the father of the people, rather than some distant, god like figure.