And what if the peaceful, deliberate split was a north/south one, not an east/west one?I'm talking about a west-east split od equals: Roma-Constantinopolis, not about a south-north split.
Britannia would be the name of a usurper breakaway state there. And then Hispania, Gallia, Oriens, Africa, etc. All based on regions, in contrast to the central imperial west-east authorities: Roma, Constantinopolis.
If the game allows for other splits than Roma- Constantinopolis, I don't have any suggestions, other than that more generically Constantinopolis would be replaced by Oriens. And still Hispania, Gallia, Oriens, Africa, etc. So regions instead. But then the name Roma stand out, in terms of consistency.
What are your own suggestions?
Calling the ERE effectively just "East" isn't brilliant either.
I'm not sure what I'd suggest overall. There needs to be something sensible so that if you get a situation like the WRE/ERE split where **both** were Rome and the split was a deliberate administrative decision, there needs to be a way to reflect that without effectively downgrading one to being known just as a province of the Empire, as would be the case if the spun off state was "Hispania" or "Britannia", and so that the name is accurate (a "Britannia" that contains Britannia, Hispania, and the Gallias isn't really that accurate). It also needs to be systematic, and applicable to *any* large empire that splits in that fashion.
The flag is easier. Make a couple of different coloured versions, so there's a red Roman banner, a Tyrian purple one, and a deep blue one. That allows a three way split without too many problems, and maybe allows for them as short-lived revolted flags as well. The same principle could be applied to the other likely large empires.
Possibly there could be something to try to detect where the split empires lie in relation to each other, and to give them a directional designator, although this could (and probably would) fail if either splits again. I don't know how effective or complicated this would be to code for, since I don't know if there's a function for "this state is in X direction from this other state", or how that would be coded for.