Ideas for rituals: Pact of Xavius, giving the completing civ a few satyr units. This should probably only be available to evil civs, assuming you're keeping FFH's alignment system. Blood of Mannoroth, giving all units of that civ a Fel Taint promotion, a small strength boost and chance to become enraged. Maybe applying to all units created by that civ afterwards too. Call of the Firelord, to basically turn your civ into a sorta fire elemental civ. You'd still be able to build all your old civ units/buildings, but could also have fire themed unit choices, e.g. fire elementals, core hounds, flamewakers. Also, this ritual would create Ragnaros (held) in the city that finished the ritual. Maybe to balance out the power and versatility of having new unit choices, your normal unit choices would be created with the Weak promotion, to show that they are enslaved. Alternatively, if you want to have a set of late-game summonable civs like the Infernals/Mercurians, you could use this ritual to summon a separate fire elemental civ. Not sure what civ you could have to oppose them though, since the water elementals don't really have a true presence on Azeroth, they just hire adventurers to do their dirty work for them.