Kind of a complicated question. What would a empire need for it to be able to conquer the world? I'm not talking about an OTL empire, I'm talking from a perspective of a theoretical one. There's a reason why some empires and nations are able to expand rapidly and some aren't. So what would an empire have to be like if it were to be given a shot at expanding across the world. What are the "all-star" traits needed for an empire to have a chance?
The world or the known world?
I'd say it's highly unlikely pre-1900 for the former. The latter is more doable, but even Alexander had to stop somewhere. The big problem is that before the modern era, most Empires' fates were tied to the ruler, and after his death centrifugal forces would overcome the centripetal ones and result in dissolution or reduction is size. Obviously there are exceptions, like the early Caliphates and Rome, but by and large, the Empires would fall well before they can subjugate the world.
As for the modern era, it would be simple. Get a jingoistic power, give them nukes, give them a monopoly on nukes, and let them go wild. They'd probably control the entire world for as long as they have the monopoly.