It's since I got into the game (now a year and a half ago) and I discovered some Faction (AI) troops that would be very nice to look at and use in player armies.
Perhaps select one for each playable race and classify it as T3 of only one troop type (infantry, archers, spears and cavalry).
For example, I would find it very useful to add Ironwalls to the infantry of dwarf players, while for elves the Arrowspeakers as a unit with the bow, for orcs perhaps you can add Orcwalls to spear units or Ogres for their infantry and for humans the Praetorians (already assigned to them but not usable).
Perhaps you could also add another branch of units like the Faction Troopers, which can only be recruited through a single unrepeatable building. Maybe a sort of "Outpost" or encampment or a Faction Embassy, ​​inside the City, linked exclusively to the type of race chosen by the player.
Perhaps you could also add the option, once the building is built, of being able to choose a Faction from among the existing ones and thus being able to have some sort of one-Faction troops from the building.
Always remaining within the concept: for the dwarves only dwarven factions, for the elves only elven factions etc.
In short, the units are many and it would not be bad to add at least one for each playable race.

For the resources and research used to unlock and recruit these troops, I think it can be developed as for any other military troop in Illyriad.
Taking as an example the units of the Ironwalls, of Clan Moedagh (Dwarven Faction AI).

Product:

Ironwalls <> 5 Gold p / h

Resources Required:

Sword: 2
Chainmail: 1
Plate armor: 2
Beer: 3
Gold: 6

Production Time:

9m (...)

Why this amount of gold in upkeep and resources in construction? Well simply for two points, 1) because to get to use these units, with a well concentrated number of attack / defense points, it must be necessary to spend, work hard for it, even cry.
2) because this kind of unit, at least speaking of dwarves in this case, must be limited, since we are talking about "heavy" units with large points and therefore can be limited to being much less than the rest of the basic units.

As far as research is concerned, I believe we must add a small branch exclusive to each race, in the military tree, which collects at least three or four searches to be carried out.

The first research could be related to the unlocking of the Faction building such as: outpost (elves), embassy (dwarves), encampment (humans), camp (orcs).
The second research could be related to the unlocking (at level 20 of the building) of the Faction troops and the third and final research the unlocking of the elite division of the unit kind.

The building has to be levelled to 20 to be able to access the research and then unlock the unit, as you have to earn the chance of a powerful troop.

As far as the characteristics of the troops are concerned, their defensive / offensive and speed / capacity score could be added or decreased for a better balance.
Personally I would see good to leave the stats of the units as they are, maybe improve the attack of the Arrowspeakers for the elves and therefore decrease their defense accordingly. To conclude, I believe that the Defense and Attack Skills can be the same, without adding any others. Two of the suggested troops are Infantry, whilst there are no Cavalry suggestions. 

At least these are the things I can talk about, in the most practical way.

Tell us what you think and please add your thoughts on the way you think the resources required should be, stats should be or anything else you can think of.

Regards

Ironhand