I must say, this is looking impressive! If I might recommend some reading material for you it would be "In the shadow of the sword" by Tom Holland. It is Historical non fiction based in the area of your mod, you might glean some inspiration from it! As for what you should play as, I recommend any of the Roman successor states (except the Byzantines) I always found them
fascinating. (also, someone here mentioned Easter eggs, so I would recommend a little reference to L. Sprague de Camps "Lest darkness fall" In Osrogothic Italy? You don't have to of course)