What is the difference between a horse and a warg for kingdom/empire factions, other than the image? This would be one basic uniqueness which is already present in the game but is ignored ... has anyone ever seen an AI with horse or warg mounted troops? I haven't.
I'm all for unique units, and it doesn't have to be monsters, It could be a unique class of character, some spells that are available perhaps to one faction but not another (the spellbooks in this game are way too similar in my opinion .. teleport should be only in one spellbook, say air, spells should have unique counters between books, i.e. the "water" book should have some specific spells that weaken fire based spells such as fog, haze, rain even ... conversely these things should "amplify" the power of electrical based attacks, such as from the "air" book because we all know water conducts electricity, and finally, earth book should have some counter to "electrical" attacks, say earthen mount, metallic rod, which would help to defend against "electrical" attacks because we all know that earthen ground "grounds" electricity). Also some specific buildings for factions would be great.
All these things would be great in theory, but in reality are probably way beyond the ability of AI programming. There's no reason to add a "special" unit if the AI doesn't use it or just uses it in a stupid way. The AI already doesn't use most of the spells it has now, so adding more would just be useless.
It's like a part of me says, yeah I'd like to see aligators, but not if the AI does nothing but charge forward with it to die, because what's the differece between that and any other mace man unit?
A unique unit needs to have an observable difference (as to/vs any other unit) in the game beyond just a jpg "image," because otherwise what's the point?