Books on these issues are not common or complete so there is a lot of room for serious study. Of course a specialty does not have to be just one emperor. Some collectors look for various examples of a type and follow its development over the years. Some rulers are best collected as a group due to they way their histories were intertwined. This certainly includes the family of Constantine or the issues of the Tetrarchy founded by Diocletian.
Greek coins allow specialty from several angles.
A common specialty in Greek coins is collecting coins perceived to be of fine art style. Mint errors are also an interesting way to form a collection, such as this brockage from the Roman Republican period. The important thing in selecting a specialty is to find a proper fit for the individual collector. Persons with failing eyesight will avoid fractional obols or die studies while persons with the ability to make fine distinctions will enjoy a 'flyspecking' specialty requiring counting the knots on the ribbon of the laurel wreath or the like. What is best for one collector is boring for the other but everything is available within the overall realm of ancient coins.