May as well get the Roman military system out of the way, and then get onto the Celts (Irish, Picts and Britons) and how they're going to function.
Codex I.c
So how to model the Chaotic mess that was the late Empire's military system? By splitting them into four main groups.
I. Legions, the Comitatenses and the Pseudocomitatenses.
These will function as vassal Military units of a Vicar (yes that's not quite how it worked in real life, but stick with me). They will be under the Mercenary tab, but only their lord will be able to raise them (of course, the Emperor can just raise all of a vassal's troops and they will be raised anyway). They will be Rome's most effective units, and what's more, they cannot be hired by anyone outside of the Empire. They may also have a role in influencing politics (in the time established fashion of getting your Legion to "spontaneously" acclaim you Augustus. All in all they are pretty useful troops, and will have their own decision menu for their Lord and for the Emperor (establish a Legion, disband a Legion, ect.)
II. Limitanei, Laeti and Provincial troops.
These will fall under the Levy mechanics. The Limitanei will be represented by having special buildings along the frontiers that gives those regions extra troops. Otherwise they are whatever soldiers you can scrape together. Don't expect them to be able to accomplish much (they will be mostly light infantry). Any Barbarian nations you subjugate will come under these mechanics as well (such as Stilicho's use of defeated Goths in his army). Numbers will be artificially kept low through a "Decline of the Roman Empire" modifier, to represent the various plagues and other factors that kept the Roman Empire short on manpower (as well as the fact that the best were put into the Legions).
III. Foederati, Barbarian Allies.
These will probably be mercenary troops that the Romans can raise. They will not be particularly reliable, and may revolt if they are not paid or just fancy some of your land. The composition will depend in which group of barbarians they are. They will not be vassals.
IV. Scholae Palatinae, Personal Guards.
These will fall under the Retinue mechanics, and will be the personal guard of the Emperor or high ranking official. You can choose the expensive Scholae or the cheaper Bucellarii (which will be less effective in battle, but more numerous and affordable for Provincial Governors).
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I think that's everything, if anyone has any other ideas, post them here.