MEGApack update.

In my opinion, the most correct approach would be to use NCF capabilities for second upgrades and alt-upgrades... I think that existing creatures from various MODs can be tweaked for this!

 

This is also easy to do poorly. 
Minotaurs upgrade... into super duper minotaurs and mega super minotaurs. I don't know who would need a bunch of identical creatures. If all this grade nonsense is to be done, then ideally they should add something new to the game. 

By the way, I mapped out an idea for myself regarding what 3rd level upgrades could be. Now I'll try to show it using the first creature as an example. The main upgrade is like a continuation of the base creature's ideas, just improved—a combination of features from the second-level main and alt upgrades. This way, those who like it can keep the familiar gameplay. And the 3rd level alt upgrade is something completely new. 

Empire 
Spearman / Wolf or Dogken Trainer

Undead
Skeleton spearman with a shield, who is simultaneously a ranged and melee fighter / Bone Spider

Dark Elves
Cool assassin / troglodyte. In the description, the transformation of a dark elf into a troglodyte can be justified by a phrase like "dark elves sometimes perform gruesome rituals and turn their servants into monsters similar to ancient earth spirits". 

Academy
Cool gremlin (more mechanic and support functions) / Gremlin Mech

Orcs. 
Cool goblin / Goblin on a Boar

Demons. 
Imp Jester / Imp Maniac

Elves
Cool dryad / Carnivorous Plant

Dwarves
I'm completely clueless here. 
Only the option of cool dwarf / cool dwarf.
The alt could be made without a shield and with a large axe. 
On the other hand, musketeers could be an alt to spearmen, and the cannon can be taken from the same siege engine used by dwarves in 7. The one in dragon form.

Added after 9 minutes
The morale here is that all of this should be done with quality, thinking through the balance and making the visuals good. And that takes a huge amount of time. Especially since I've only mapped out the first creatures. 

Take skeletons for example. When creating 3rd level creatures, you'll want to use models from 7/6. But they will clash strangely with models from 5. Therefore, ideally, visuals would need to be updated for all skeletons. And it's the same for every creature. A huge amount of time. A huge amount of effort.

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But, of course, many interesting options can be thought up here. 

For example, alt mages as something between a Sword Mage and a Magic Archer

Alt titans: Dragon Golem

Alt elven dragons: Dragon-snake shifter, like in dmc4