I wasn't aware of that the "faction slots" for the Roman factions had to be allied. This of course makes my suggestions wishful thinking. Anyways -

My suggestion:
1. S.P.Q.R
2. "The Rest of the Romans"
3. The Mauryan Empire (in India)
4. Pergamon, Illyria, Masilia or the Samnites (can't decide, each would be interesting in their own way)