hmm so when i take the darkelves heroes and commanders as example (but as all armys have more or less the same structur it would work for all):
malekith and morathi are the leaders.
heroes would be:
helebron
lokhir
malus
shadowblade
kouran
tullaris
and the minors, great commander style:
master,dreadlord.
and the sorceress, supreme sorceress are the mage units.
--------------------
as you dont want too much equipment choices in the mod, the ones with the most equipment choices in the tabletop are used as minor heroes then.
maybe you could even use both as great commander:
first you can just get the "master" great commander and after i.e. half of the military tech tree or after a special research is done the great commanders get a bonus when they are attached to units... i.e. master +5% strengh, after research its 10%.
just as example to show what i mean... so after a while your civ knows how to train better great commanders. (maybe 2 promotions could be used there. the "master attached" promotion (+5%strengh) that the unit gets when you attach a great commander. and the "dreadlord attached" promotion (+10% strengh) would be used when you have research XY done)
----------------------
leaders and mages are not mentioned now... it should be clear.
----------------------
and these "special character" heroes usually already have unique equipment where one of these items could be used then. or a special ability (spell).
i.e.:
shadowblade:
like assassin, +1strength (like potion of strength), +1 firststrike (master assassin), enemy -1 firststrike (hand of khaine), when dead all units 1field around the shadowblade stack get damage (heart of woe).
item: potion of strength or heart of woe.
obtainable: same as assassin.
costs: 4x assassin costs.
lokhir fellheart:
like corsairs, +1 firststrike for each 3 units in the enemy stack (red blades), 10% chance to replace the attacked friendly unit (dareing leap), regeneration promotion (krakenhelm, i guess better no terror too or?), +25% chance to capture a defeated enemy unit as slave (merciless slaver)
item: krakenhelm or red blades.
obtainable: same as corsairs.
costs: 4x corsairs costs.
malus darkblade:
same as cold one knights, -1 enemy strength (warpsword), cold ones in same stack are not stupid (if they are stupid in the mod) (spite),
"tz arkan" spell: lasts 10 turns, +1 strength, +1 firststrike, if malus has not attacked this turn, one random unit in his stack gets 10-90% damage.
item: warpsword
obtainable: same as cold one knights.
costs: 4x cold one knights costs.
hellebron:
same as witch elves, -1 enemy firststrike (parrying blade), +1 firststrike (rune of khaine), 50% chance to ignore first spell damage each turn (amulet of fire), +2 strength (deathsword), all witch elves in the same stack get +10% strength (queen of khaine).
item: parrying blade, amulet of fire, deathsword.
obtainable: same as witch elves.
costs: 4x witch elves costs.
kouran:
same as black guard, +1 strength +1 firststrike (crimson death), if attacking enemy has less strength then kouran= 20% chance that attack fails (armor of grief), black guard in the same stack get +10% strength (black guard captain).
item: crimson death, armor of grief.
obtainable: same as black guard.
costs: 4x black guard costs.
tullaris:
same as executioner, +1 strength +10% strength no first strike (blade of har ganeth), causes fear (tullaris the dreaded), if tullaris kills an enemy unit= 10% chance to attack again (sacred slaughterer).
item: blade of har ganeth.
obtainable: same as executioners.
costs: 4x executioner costs.
------------------------------------
all items and special abilitys are promotions (i.e. hand of khaine, sacred slaughterer, red blades)
all heroes have unitlimit 1. (no 2x shadowblade). maybe even limit it to "one on the map". so that if 2 civs are playing as darkelves and one has builded i.e. shadowblade the other cant build him too. (a bit like wonders, you get a message when a hero got build and you can change your production when you build the hero too).
all heroes can be built again 10 turns after death. (they are heroes they get heavyly wounded instead of death)
unit who has killed a hero gets one of the ITEMS the hero carrys. (as promotion, item will be randomly chosen) this item cant be traded and when the unit dies its lost. (yeah its possible to get i.e. 4 units with helebrons sword when you beat her 4 times, but as most heroes have atleast 2 items and because it taked 10 turns again to build her it might not happen that often.)
------
awaits the usual "too much promotions" "too much heroes" stuff from Ahriman.
no idea if one of them got too strong... i tryed to design them so that they are abit like the tabletop... i think helebron has some abilitys not (witchbrew) but i guess she doesnt need more...
i think each special char of each army could be designed like this.
but i think it would be worth to include all special characters in the mod. i think in all armys they could be seen as "hero of a unittype" (i.e. hellebron leader of the witchelves). so they are buildable when you can build the unit but takes way longer to build.
i think each hero of all armys have unique enough items and specialitys that they are not repetive... and these items and abilitys could be given with promotions.... ok there might be a very long list of promotions when each hero of each army has more or less 3-5 unique promotions that will just be used for that one unit... (bad performance result?) but it would add alot warhammer flair if all special characters can be build.
of course, first there could be just one like in FF, but over time all the others could be added.