I'm thinking about giving a go at the magic of E:wom, as that seems to be one of the static areas in the coming patches. I would be very glad if people could read through this and give feedback, both on the mechanics and the 'fun' of such a system. There is a tl;dr at the end.

-Spells deal damage and cause effects according to the caster's intelligence, and occasionally attack rating (melee magic).
-Half of your sovereign's intelligence is also added, even if the sovereign is not present. (not possible)
-A mighty sovereign that invests in intelligence and magic need not worry that his subordinates are not intelligent enough.
-Essence is reintroduced as a per-combat limit on spells. Mighty spells require more essence drained. Sovereigns alone have an infinite amount.
-Only native channeler champions and sovereigns regenerate essence during combat, imbued champions will not. Note that essence only slowly regenerates in strategic combat (3 per turn).
-Imbue Champions provides a minimal amount of essence, the amount of essence is increased as caster/sovereign intelligence increases.
-Spells no longer have an intelligence requirement, instead spells have a desired intelligence - casting with a lower intelligence reduces damage and casting with a higher intelligence increases damage.
-All elements provide damage spells, and other spells. Some spells drain the caster's damage bonus, others negatively affect the target's resistance.

Damage is calculated from a base value of 1 = 100% core damage.
The 1 value is affected by modifiers below, that stack, so a caster in a desert will deal 1.25 = 125% damage with fire spells,
but only 0.75 = 75% when casting fire in a swamp.
A fire caster in a desert will deal 1.5 = 150% damage.

This value is then weighted against an elemental resistance value, also by default 1 = 100% damage taken.
Resistance values are inverse in power, ie 0.8 is stronger than 1 and means that the unit only takes 80% of the damage calculated from the caster.
For example, a caster may have an affinity of 1.5 = 150% damage, dealing 10*1.5 = 15 damage.
If the target has a resistance of 0.8, this value is then reduced to 80% (15*0.8 = 12).

Terrains: 25% bonuses/flaws
Desert: Increased fire damage, decreased water damage
Swamp: Increased water damage, decreased fire damage
Forests: Increased earth damage, reduced air damage
Hills: Increased air damage, reduced earth damage

Environments: 20% bonuses/flaws
Fallen boosts Death Magic
Grassland boosts Life Magic

Death/Life affinity will come from cities. Higher level cities will give higher affinity.

Units in cities gain no bonuses from terrain, but they do gain bonuses from the environment.

Buildings:
Elemental Affinity buildings give boosts in a city (one per each type max, levels with city 3-> 5-10-15%)
Shards boost the connected settlement (40% bonus), also protects against the type (20% resistance).
Shards also give a global bonus to the affinity (10%), completely replacing the old shard mechanic that adds a static amount of damage.

Champions:
Some are native spellcasters with high essence and intelligence.
Some are not spellcasters and have no essence, but may still be intelligent.
Regardless of being a spellcaster or being intelligent, some champions have native elemental affinities of different strenghts, for example being a fire caster increases the fire damage from that person by maybe 25%.
Similarly, some spellcasters have resistance against different types of damage.
Exceptionally strong and rare champions may boost certain elemental damage for every caster in the army, potentially creating a coven or school of casters.

Items:
Magic items of various types provide minor bonuses to a caster, and can provide resistances as well.
Some rare magic items also increase the essence regained per turn in tactical combat, allowing for more frequent casting.
Items may also "imbue" a champion with additional essence, turning a non-caster into a caster, and a caster into a more durable caster, however these are exceptionally rare.
Items may grant you resistance, but also take away your power in the same element, or another element. Others may grant you bonuses, but take away your resistance. Some may grant you resistances in one area and take it away from another.
The mythical 'Tear of the Gods' grants almost full immunity from spell damage, but also causes major incompetence when it comes to casting.
Rare tomes may be found that train the user in a certain element, but these bonuses are very minor.

Creatures:
Most creatures are now resistant to some types of magic.

Note: It is possible to become immune to certain spell damage, possibly even to regain health if the enemy casts a type on you. For example, zombies and ghosts are known to regain health from Death magic.

tl;dr: I want to reintroduce essence as a per-combat limit on spells. I want to spice up the mechanics of magic by providing different sorts of bonuses to magic power, and introducing specific resistances. Bonuses and resistances can be regional based on your location of the world map, or based on a champion's history, or the items he is wearing. Creatures may be natively resistant to certain types of damage.