I think that the Infernals should get access to some guilds maybe a arcane guild that uses demons to make money and magical stuff.
I also think it would be good if you purchase units from the guilds. Depending on what guild you have active will depend on what promotions it starts off with. Buying a mage from an arcane guild that likes demon and undead will have promotions like death and entropy while a guild that dislikes demons and undead might give you a unit with life and nature promotions.
This would work like the hire mercenary button that's in Rhye's and Fall of Civilization or the ship units from off world button that's in Dune Wars. You could see the units and what promotion they have and buy the ones you want.
I think that it would be good for there to be a good, neutral and evil guild for each type of guild. Good guilds will only make money by honest means. Neutral guilds just want to make money but wouldn't commit crimes (or in the case of the thieves guild kill. They just want to steal) do make it but at the same time they would not go out of their way to help people. Evil guilds have no morels they will kidnap, kill, steal, summon demons anything to make money.
That's my idea anyway, probably will come up with some more later. Keep up the good work.