Names for the romans should be thought carefully too, having a point of view that spans also into part 2. "Romans" is too generic if there are planned at leas 2 more roman civs for part 2. I would go for "Roman Republic" for the part 1 romans. I would propose a classical "S.P.Q.R" for the High Empire and "Imperium Romanum" for the Late Empire. I wouldn't make any distinction from the east or west late roman empires in (in terms of having two separate factions) because they were mainly the same, except perhaps for the clibanarius and some minor differences in the unit skins, to solve this problem i would go for a AoM aproach (like my mod): When you are using late romans, and when you pass to the third phase, you can choose if you want to be a "governor" from Constantinoupolis or from Roma. If you choose Constantinoupolis you would gain access to the Clibanarius and the Syrian archers, and your legionaries (limitanei, comitatenses, etc) will be skinned in the eartern fashion (and perhaps a couple of techs); if you choose Roma you would gain acces to the Foederatus and the Vandal corsair ships and your legionaries will be skinned in the western fashion (and again, a couple of techs). I think that way is better than having two late-roman empires with just minor differences in the units