RTF is a global modification that replaces all towns in the game.

for example, humans will have angels and dragons – but what if we can transform an angel into a dragon and vice versa in the creature upgrade window (like we do with regular and alternate creature upgrades)? That would clearly be bad.

Maybe if we don't make the angel a "pair" for the dragon, but simply place them in the same building, there won't be any problems? The simplest solution is to make the cost for retraining a 7th-level creature a huge amount of resources and gold :D XD

Added 2 minutes ago

When we were discussing "symmetry" with you, I was confused by the fact that in the case of Campaigns or story maps, when there are events or scripts tied to 7th-level creatures, we will get some kind of nonsense (or some kind of inconsistency with the plot).

In the Necropolis Campaign, in the 5th mission, when we captured human cities, we could only recruit the second upgrade, but not the first. So I don't think there should be any problems.

Added 2 minutes ago

I think they would go for the Necropolis if they were green.

Not necessarily green... and... what is that? Are you kidding? Female vampires? 0_o I think it's better to take a vampire from the 6th level.

Added 8 minutes ago

I didn't understand about the building; they already upgrade a specific creature.

Well, look: In Heroes 2, there is a fort on a hill, but it only upgrades ogres to ogre lords; it doesn't upgrade spearmen to veteran spearmen, etc.

Added 2 minutes ago
And here's another idea: I think it's better to give "training" to the Lizardmen, and give "blood rage" to humans – but call it something else, and let it give points for defense?

Added 1 hour 45 minutes ago
For the 7th level, you can add Wendigos to the Mages' Chimera.

Added 2 hours 19 minutes ago

I would just increase the required level.

Yeah, so the first option, but with increased requirements in the form of city level? Okay. :)