Okay, sorry about the confusion, I was writing in haste. 

I'm not all keen on specifying an absolute date. Doing that confines me to strict historical accuracy and all that comes with that. The role play will be following its own story and characters, simply based at a time when the Westerm Roman Empire still occupies Iberia, Gaul, Italia, Northwestern Africa, and Britannia. 

I'm not sure if you've ever seen that King Arthur movie I linked, but it's about Artorius Castus who commands a Sarmatian auxiliary unit in Roman Britain, and he ends up fighting the Saxons. That movie provides a really good example of what I'm trying to achieve: specialized Roman Auxiliary units working for Rome. The difference is that they're not all Sarmatian.

At various points the players will run into Rome's enemies and may have a chance to help or hinder them. Helping enough of them will eventually bring about the downfall of the Western Empire. Hindering to many of them ensures the Empire's survival. However, each choice tests the player's loyalty to his people, the Romans/Marcus, and Arcani X.