You can increase or add law and happiness bonuses to your faction's buildings (export_descr_buildings file in data folder). For example:
Code:
building core_building
{
    levels governors_house governors_villa governors_palace proconsuls_palace imperial_palace
    {
        governors_house requires factions { saba, slave, barbarian, armenia, romans_brutii, egypt, romans_scipii, carthage, parthia, numidia, thrace, greek_cities, macedon, romans_julii, seleucid, pontus, }
        {
            capability
            {
                agent diplomat  0  requires factions { barbarian, armenia, pontus, romans_brutii, egypt, romans_scipii, carthage, parthia, saba, numidia, thrace, greek_cities, macedon, romans_julii, seleucid, }
                law_bonus bonus 3 requires factions { armenia, pontus, }
        law_bonus bonus 4 requires factions { "EB name of your faction here", }
happiness_bonus bonus 1 requires factions { "EB name of your faction here", }
                recruit "generic militia" 1 requires factions { saba, slave, barbarian, armenia, romans_brutii, egypt, romans_scipii, carthage, parthia, numidia, thrace, greek_cities, macedon, romans_julii, seleucid, pontus, }
            }
            construction  2
            cost  800
            settlement_min village
            upgrades
            {
                governors_villa
            }
        }
1 stands for 5% bouns and 4 for 20% (I think).

Unfortunately, the problem with this solution is that you get extra money from your far away settlements because of higher stability. And if you overdo it your game will become trivial (tons of money and no revolts) in a very short time. In short, it unbalances the game.