Reddit reviews Museum Careers: A Practical Guide for Students and Novices

You will discover what specialization you want to choose after you start studying history more. There is really no other way. And as RedPotato mentioned, there are various curators in larger institutions that have pretty specific specialities, and Roman history could fall into one of those curatorial specialties. You could have a PhD in something like Material Culture of Ancient Rome, but it will be extremely specific. With that being said, having such a specific specialty is also limiting. You may have to move across the country to fulfill a job, or be very patient until something opens up. I also want to stress again that there are numerous other positions in the museum field other than curator and director and if you're interested in museum work you may find that something else suits you better. But you won't know until you do more research, meet people who have jobs in the museum world, and try out museum work for yourself. Get a hold of the book Museum Careers: a Practical Guide and read through that. I think it will give you a better sense of what to keep in mind going forward with your interests.