i would think it would be a faction that would last for maybe the entire campaign or have some major impact on history

i would hope they would add a rebel romanii faction
that only appear when scripted (CIVIL WAR)

a Numidian faction would be good, to control cathage expansion
parthia would be good?
i'm not so sure about my eastern empires but does Parthia exist already in EB?

anyways

1. Rebel romanii faction
2. Numidian faction
3. 2nd germanic faction