A massive Ancient Rome Sims/Total War/KotOR type thing. Create your character, then immerse yourself in all the loveliness the Roman Empire had to offer. Work your way up the cursus honorum
to become Consul, or go out and fight as a soldier, etc.
Also be able to choose the location of your residence and tailor it to your own wishes, be able to move across the Empire and fight to expand it.
And, of course, be able to work your way through the entire
history of Rome. Just over a millenium of history isn't that much to ask, is it?