It's also the beginning of when cracks began to appear in the Roman Empire and the rule of Commodus is often refered to the end of Pax Romana.
Not a specialist in Rome at all, but in our Empire, the cracks didn't just BEGIN to appear - they are already present, and getting worse. So more like the Severi, I suppose.
I support Das's idea (I think it was Das) of having players create characters as various members of society. People shouldn't just attempt to be kings, or soldiers. A minister calling for change in a stagmenting government could be a powerful force if s/he has enough charisma. Likewise politicians, political rebels, and other non-military members of society could play a huge role in the story. Imagine something along Ghandi, Boris Yeltsin, Nelson Mandela, or others standing up to the government and leading massive protests. On the other hand, characters could attempt to take over the power of a nation the likes of Hitler, Stalin, Mussolini, or other dictators.
My favourite idea is the various "well-meaning" societies - both reactionary and revolutionary, and, ofcourse, the Masons.
I believe religion could play a major part in this nes so a couple should be created.
I'd say it probably could be something of a Romanojapanese Emperor cult. The Empire must have an Emperor... even if he is a figurehead.
The empire should have land on every continent, but rather than dominating everyone the empire should be held back by weak alliances.
Its not much of an Empire if it can be held back by some weak alliances.

No, the Empire must be hegemonic. There could, would and should however be some small and medium-sized powers waiting for it to collapse.
he main strenght of the empire would be its ability to mobilize rapidly and a very large percentage of the worlds population.
The strenghts of any real empire are a good strategic position, vast resources (of all kinds) and the ability of projecting force in any position; furthermore, it needs the willpower and organization to do so. And, ofcourse, no empire rose in vacuum - they all rise when everybody else is too weak to stop them from doing so.
However, our Empire is overstretched and stagnating, and hated by the others; oh, it is also feared ofcourse, but as soon as things begin to go wrong for it the fear might disappear, and being neither feared nor loved is a rather unpromising situation...
The Empire itself - I've been thinking of the various models, and, as already hinted in this post, perhaps it could be something like pre-1945 Japan combined with Rome under the weaker emperors? Basically, a figurehead revered emperor dominated by various military strongmen and politicians.