There was indeed an expedition to Parthia planned to avenge the death of Crassus at Carrhae, but Parthia was too big to hold onto so I suspect Caesar would have settled for just Mesopotamia. Rome had enough trouble holding on to that later although Rome is admittedly more dynamic at this time than a few centuries later while the Parthians are not (when compared to the Sassanids). In spite of the political crises and civil wars, the 1st century BC still had economic growth. I think Caesar could have taken Mesopotamia, but those after him have to hold on to it.
If the Romans can hold onto Mesopotamia, it would weaken the Parthians (or whatever incarnation of the Persian Empire succeeds them). The region is an administrative centre with many cities (Babylon, Susa, Ctesiphon, Nineveh) and a big economic centre since trade routes run through here not to mention the fact that the area around the Tigris and Eufrates rivers are agricultural areas of big importance. The permanent loss of such a vital administrative, economic and quite densely populated region could render any eastern threat to a mere pest at worst or make the Parthians a Roman puppet at best. This secures the eastern border. If/when the Huns come, the Romans might be able to resist them better.
Then Caesar can turn to Germania and establish Eurofed's fabled Vistula-Carpathian mountains - Danube delta/Black Sea border

. This could be done if Caesar had his hands free to conquer Germania.