Did the Roman Republic/Empire have a set career track for its soldiers to follow? How did they recruit, train, and assign new soldiers to the legions? And once you joined your legion, and assuming you survived long enough, how easy was it to rise through the ranks? How typical was it for a poor recruit with no political connections (at first) to become an officer? Were there any instances of Roman soldiers starting out as a rank and file recruit but later commanding a legion? How were promotions handled? Were there any rules as far as time in rate or written or oral exams the soldier had to pass?
I just thought of something else to add: would a soldier spend his entire career in one legion? How often were soldiers re-assigned to other legions and/or transferred to a different part of the Empire? For instance, in the modern US military typically soldiers receive new orders every three years or so and are sent to a new base.