Although I understand that some people may want some cool mechanics for the Imperial Legion, IMHO CK2 core mechanics are more than enough to represent permanent forces of great empires using retinues. I mean, obviously the composition of the armies is up to you, but nobody said anywhere that the Imperial Legion itself didn't change over time; it's just a concept. Playing Elder Kings I managed to increase military technology enough to have several "legions" (I made a standard composition, just for RP) deployed wherever needed. As a matter of fact, I did this not only as Emperor of Tamriel, but with other imperial titles as well (my battlemage aldmeri legion was ultra effective).
Maybe I'm a bit conformist, but I just fail to see the point of trying to implement some overtly complicated mechanics for the Imperial Legion when you can have any "legions" with any country as long as you invest resources, time and tech to get good retinues. If anything, military/crown laws could somehow give more retinues at the expense of levies to represent a more centralized army for Imperial titles. I think I did a small personal mod a while back doing exactly this, and it was fun.