If you use a combination of spies and assassins, you might be able to get the roman settlements to revolt. The city pretty much has to be away from the factions capital, be big enough to have some squalor, small enough garrison that it will revolt if the public order drops, and even then it isn't guaranteed to work, but if you keep it up it will happen. You dump several spies into the settlement to drop the public order down as low as you can get it, then have the assassins sabotage every building that gives a public order bonus. Hopefully the reduced public order will cause the settlement to revolt. Even if you don't or can't go in and grab that piece of territory for yourself, it gives the roman's enemies a chance to, and if not the romans have to divert attention to retaking it.