In addition to guilds, I think we need proper religions. At least 4 elemental religions, and perhaps a religion for death, and possibly an order-eque religion for life? alternatively your life religion could be more "nature based" although I would think earth, wind, and water have plenty of chances for "nature" ... especially EARTH.
meanwhile, there should be room for other various cults and religion ...
For instance, the main Death religion could be Fiends of the Fallen, a fascist, tyrannical, sacrifice the weak and abuse the poor, place the powerful,greedy, and evil up on pedastals and heroes ... and above all probably some sort of hybrid-study (more mutations) or demon worship of some-sort (probably parasites "item" that lives in the host fallen, giving them extra powers). This "Main" death religion would be only allowed by either a fallen sovereign or a fallen nation. Power over Free-will.
Meanwhile, as there are parts of the Religion of Ruin as it were, which involves re-animating the sacrificed slaves for minor minions to aid in combat ... a more "undead" based death religion could be adopted by either race (more preferably evil-aligned humans) that do not believe in the honor of demons, and think only science ... science of the arcane arts of necromancy ... will lead to the ultimate truths. Therefore, most of the army is built entirely around the undead ... of course zombies will be aplenty, although you can also acess the rare skeletal beast, or bone horror ... although to keep in its favor you will have to have a steady stream of bone meal made from your dead (you have a meter ... taken from "sacrifices" from the cities and "casualties" of war ... which represents the amount of dead bodies you have acess to. Bone Horror would eat perhaps 5 bodies per turn? 10? can devour extra bone-meal to heal faster)
grunt troops would have religious zealot "taskmasters" which means the dark priests and their mortal, fanatical, (brainwashed?) footsoldiers. Who relay orders to legions of skeletons, zombies, and possibly some beastly horrors.
Special abilities of dark priests would include fear tactics, level drains (wears off after battle ends/or alternatively at the end of the world-turn* "whichever is longer"), and perhaps imbuing their undead, occasionally mortal soliders too, with diseases designed to spread quickly over the battle-field and kill its host fairly quickly ... although surviving the host just long enough to best enable full battlefield spread (several hours) ... and perhaps a certain ability or spell (or soveriegn spell) that causes all currently diseased (living) units to die instantly, the down-side being that the disease is also wiped off of the battlefield.
Im thinking the Zombie/Skeleton Necromancy religion should have access to the second best diggers ... ghoulish tunnelers and gravediggers ... second of course to the main Earthen religion (and any digger based Earthen-spinoff cults). If there is some sort of poisonous and bloated Death Grub/ Death Worm ... perhaps it could be the exclusive unit for the leader of the Necromancy religion (kind of like a unique patriach unit .. only a monster)
Which brings the issue of religion spread, holy cities, fractures within the church, conversions, inquisitions, patriarchs, any sort of senate/college of cardinals .... and any other way to distinguish who commands the religion (the head or highest authority) and those that follow the religion (or happen to have sympathizers in the citizenry)
The Fire based religion could be called Firebrands or Order of the Flame (something of the sort) I bet the Fire religion should have the highest bonuses in missionaries and inquisitions (passion of the fire) so probably the religion to spread the quickest. (perhaps with some fitting penalty tacked on ... like weaker holy city, or best Holy City bonuses/ spreading/ inquisition however the penalty lies in increased worldwide maintanence) Perhaps access to certain fiery beasts (demonic one could say) like the typical Hairy beasts with hooves and horns ... or simply giant hairy bulls combined with scaly horned creatures wielding swords.
A likely spin-off of the Fire religion would be a much more reclusive/isolationist version which tends to worship dragons ... perhaps red dragons in particular. This would focus less on the passions of mass-conversion, or fiery beasts ... and focus more on age, skill, and learning through experience. Religious units and units influenced by the religion would perhaps have increased upkeep yet much faster experience gain. Dragons or dragon-kin would of course be immune to this effect, but you don't start with any way of gaining dragon-kin to begin with ... the religion gives you bonuses of dealing with such units, and grants you a greater ability. (any dragon to enter your army would be blessed with morale bonuses all around, for itself and the soldiers)