So, my knowledge of the ancient Roman Empire is pretty sketchy, but from a quick wiki trawl, I think we could adopt a version of their system. Obviously it wouldn't be right away, but if we plan on conquering more, like I think we do, it is something to think on.
There would be three classes. Citizens, Freed-men, and slaves.
Citizens are exactly what it sounds like, those who are full citizens of the empire, and are afforded their full rights.
Freed-men are those who used to be slaves, but were freed through one of the mechanics listed below.They would have many of the same rights and opportunities as Citizens, but would be held back from positions of high authority.
Slaves, are, once again, pretty self evident. However, I'd like to see if we can set a precedent against slaves as personal property, and firmly for them being the property of the State. This would greatly limit the chances for the system to be used abusively.
Now, in terms of class mobility.
Slaves would be made up of the conquered, or those who committed a crime beyond a simple fine, but not worthy of execution. There would be a hard limit on the number of years a person could be a slave, and a slave can be freed either by serving their full sentence, or performing in some extraordinary way so as to earn their freedom.
All freed slaves rise to the class of Freed-men. Any child of a slave is born into the Freed-men class. Freed-men can rise to the status of Citizen by serving a certain amount of time in the military, buying it, or performing some deed worthy of advancement. Any child born to a Freed-men will be a freed-men, excepting a family line who has been of the freed-men class for three generations. In that case, the children are born Citizens.
All children of Citizens are born Citizens. A Citizen can be lowered to the Freed-men class for committing a transgression lesser than that needed to become a slave, but beyond what can be payed with a mere fine, such as showing cowardice on the battlefield. Citizens are subject to the same laws as all others, and can be lowered to the class of slave by committing a grievous crime.
When a Child's parents are from two classes, the child will be afforded the right of the higher class at birth.
These are just my first thoughts, if anyone has any criticisms or feedback, I'd like to hear it.