EU4 isn't so much about the Ottomans as you can say that Imperator's time period is about Rome or the Diadochi. But it certainly is probably the single most influential, just for the concept it represents. With European nations having finally found their feet after the instability of the middle ages, many nations find themselves with an almost insurmountable foe in the east. But it's that breakpoint in technology and information that the period encompasses. When some states begin to explore and discover far more possibilities than were available to say, Rome when they were faced with their challenges.
So I'd say the Ottomans are the defining nation of the time period, but not the stars of the show. So to speak.
To actually address the OP's question, and to kinda reference the above point. I think it'll be both. The comparison to other time-periods don't really apply.
Rome's success and legacy were so tremendous that they've been worshiped by so many for such a long period of time. In the later years, even the most differently organized states and the most bitter of rivals would share an appreciation for and an ambition to reach towards the idea of Rome.
So as opposed to EU4, where what would become modern nations are already defined. So you might have Polish players cackling maniacally as they bully a fledgling Russia; while some players might not care for eastern Europe and fixate on colonial ambitions. But in the time period of Imperator, I think everyone playing wants to either Rise as Rome, or emulate its success as other nations. Either way, eyes are definitely on that single nation and/or its legacy.
So I think the answer to your question is both, at the same time.
Though I will also say that I think even without context, viewing the game without history's lens; Rome is pretty well situated to offer what most people seek in grand strategy games.
Large enough to begin shaping and directing in different directions from day one, small enough to seem like a challenge, with serious rivals down the road. But with enough roadblocks to keep things interesting.