I like the idea of the legion having mandatory conscription, while also allowing citizens who have served their time in the ranks the choice to either remain in the army or return to their family business. The legion has a vast territory with a large population, and cities require a large workforce. female/slave populations alone would likely be insufficient to meet the nation's demands + i think it would be pretty looked down upon for noble women to work.
I do remember writing about the legion having a working-class population alongside legionaries, as well as the existence of mass production involving manual labourers. My idea is still the same, raiding alone cannot supply the entire army alongside civilian cities.
ok so back to mandatory conscription. the male population of the legion can be categorised into two groups: free-born civilian men and slaves. The ones that fall into the latter category really don't have a say in their fate. Free-born men however when they reach a certain age (i'd imagine in their teenage years) are taken into mandatory training if they meet the necessary requirements. they must serve a period in the army and after completing their service they have the option to either continue their military career or return home. yeah, i think this would create very interesting dynamics in the army.
imagine you are a boy from the city, you likely received a moderate education and grew up in a fairly peaceful urban life. you turn 16 and the red-clad officers come to your home. you have always admired them, how strong they look. back when you were little, you would play with wooden swords and toy guns alongside your friends and beg visiting soldiers to tell stories about the glorious battles they have fought against "uncivilised" tribes and NCR profligates. you always wanted to be a soldier. maybe someone in your family, like an older brother or an uncle, inspired you. your father always told you that life in the city is better. you always thought he was a coward.
the officers come to your home, they have visited your friends too. they deem you fit for the army. on your last day before you are taken into training your family has a nice dinner together. that night you can't sleep from excitement. you hear your mother weeping softly in the next room.
you finish training and are assigned to a contubernium. you are sent to the frontlines. there, you first realise what war truly is. you secretly cry yourself to sleep on the first night. you don't like it in the Mojave, the weather is too hot and the camp is too crowded. the profligates have louder, bigger guns and your decanus doesn't tolerate failure well. The boys at the fort are all the same age as you but they are different. they had lived harsher lives, they have been fighting since they were first taken from their tribal mothers and after a decade of bloodshed and war the wasteland had made them colder, crueller. There is a fourteen year old boy with the eyes of an old man, you'll watch him step on a landmine a few months later.
after a while you get used to it. you adapt, you learn to be cruel. you learn to ignore the wails coming from the medic's tent, you learn to turn the other way when you see a slave pleading for mercy. you learn to never get attached again, you can't lose friends if you don't have any, you learn to not flinch when someone drops dead with a bullet to the head. you kill many profligates, maybe even your own brothers-in-arms, if your survival demands that. you are in your early twenties now and you don't even wash the blood drying under your fingernails anymore. and when your duty is finally over, the officers will ask you with a patronising look whether you want to go back or not. in the mirror, you'll see a scarred face that looks forty instead of twenty-two. you'll wonder how grey your father's hair is now, what colour your niece's eyes are or where they buried your mother. the red men will ask you again whether you want to go back home or not. you'll tell them what you want.