I disagree with the last post. While what you actually end up doing will depend on a variety of different factors (Grad school program, grad school advisor, diss. topic, jobs available, type of university you end up at (R1, SLAC, CC etc.) you could very well teach nothing but Ancient Roman. Its a fairly viable field; I'm interested in Ancient Celtic/ Modern Irish history and I know I'll have to go a bit broader (as in British history with a few survey "Europe Since 1715" type classes thrown in), but Ancient Roman is a really good, popular field. You may want to become a Classics major and get a PhD in classics if thats really your deal (ie learn Latin, Greek and focus on the ancient histories and languages). And you'd be flexible; most universities have both a history dept and a classics dept and depending on how much Latin/Greek you want to use you could do either.
So that's the good part. The bad part is that academe is a crapshoot in terms of jobs (in history there are twice as many PhDs as jobs, sometimes worse [has not been better since the 1970s]). You're more than welcome to join the community of "Future Homeless Historians" living under a bridge/overpass that I'll likely belong too, lol.
I suggest reading the Grad School Advice section of the Chronicle of Higher Education, especially the heart-rending Thomas Benton articles, if you're serious about this. It isn't a path you should commit to lightly. It's like getting a "break" in Hollywood or Broadway. There's a lot of hardship and very few people get rewarded; until your tenured (think age 45) you'll likely be living off Ramen and working something like 70 hours a week (teaching, writing, researching, grading, office hours, etc).
I'm not trying to dissuade you from pursuing this; on the contrary I applaud everyone who wants to go the distance. But you seem like your just getting into this, so you need to know up front that this life isn't what we tend to think it is. Many people end up burned out and bitter because they didn't know what they were getting into. But once you've sufficiently educated yourself on the ins and outs of academe (do this WELL before your junior year of college if at all possible), and you think you still want to go for it, you'll be better off and more able to roll with the punches.