Never!
I'd (and I think most of the EB team, if not every EB member) would dislike it if Roman rebels just suddenly came out of nowhere, even if Rome itself had been destroyed by Carthage, Macedonia or the Gauls. I'd say rebel faction stacks can be utilized just as well, and we could add more factions instead.
Start or hope for someone to make a Rome-focused minimod instead for EB2.