I don't know the exact mechanics of how the Roman rebel factions work, so I'm not sure if it would work to have one of them replace the other. What you could do, most likely without producing issues, would be to replace the Romano-British faction with the new one you want to introduce.

That's a good suggestion, however I really want to keep the Romano-Brits in there. I'll keep on searching.

UPDATE: it works, it works!!!
The Eastern Roman Rebels are now the Romani Pretenders, and the Western Roman Rebels are out of the game. I just added the shadow faction trait twice in descr_sm_factions.
The great thing is now I can add the Moors and rename them 'Arab and have them harry the Eastern Roman Empire from the start, making them a little less prone to domination.

UPDATE 2: There is a downside to this, which I should've thought of, of course: when one of your provinces rebels and defects to the Romani Pretenders, and they can't get the rebellious city under control and it rebels again, there's a 50% chance it will revert to a different Roman faction than it originally was a part of. An example: all the cities of the Iberian peninsula rebelled against the WRE, but when the loyal citizens of Corduba rebelled against the Roman Rebels, there was a loyalist revolt and the city went to the ERE!!! Lol. Gotta find something for that...

[This message has been edited by jgenard (edited 10-07-2019 @ 04:27 PM).]

Glad it worked out in such a straightforward way.

Another faction you could pick to remove would be the Visigoths. I've definitely encountered them a time or two, but not often or for long.

No no no! =)
They were hugely important historically, so I removed their connection to the Goths, placed them on the map in Campus Sarmatae (renamed it Aurjahaims and the province Auwjom, after the mythical Ostrogothic capital Arheimar in Oium) and moved the Sarmatians north. They now immediately fall to the Huns (after the Roxolani) but then they can usually horde pretty well and wreak some havoc in central Europe.