Some unknown thing – it's the Avatar, it's already appeared on the wiki)

Oh yes, that explains a lot!
I thought it was an element of magic, but I wasn't sure, so I made a joke. But, well, it's a beautiful, pompous, incomprehensible name for a beautiful, pompous, incomprehensible thing. Okay.

The exhibitionist is Starchild (I only associate this name with my favorite guy from KISS, he doesn't seem to have any mythological or D&D associations).

I also thought that this guy could be a good redesign for an element of magic, hence the joke about the element of exhibitionism. But alas, it's just a beautiful, pompous, incomprehensible thing. Okay, at least it's really beautiful and pompous.
By the way, where are the elements? This is also a basic thing for the heroes. But there are none, not a single one, at all.
P.S. It reminded me, I need to listen to KISS sometime...

The knight is a dragon slayer, taken directly from Might and Magic.

Well, I haven't played Might and Magic, maybe he really is some iconic unit/character there. But I didn't think he was a dragon slayer, just some super generic guy in armor. They could have made the design a little more interesting, added more dragon elements to the armor or a bigger, more pompous sword, I don't know.

I can only agree that they reused the model of the black dragon, it's like an "onyx" dragon.

I remembered my favorite onyx gargoyle from Disciples... It could have worked if they had made radically different models. But as it is, it's better to bring back the azure one, everyone will thank them for that. Azure dragons are a classic.

and I don't know where the "blue Pikachu" came from in Heroes....)))

This is such a fresh, original, and unusual idea from the Anfrozenovs, a new look at what units should be in the series. Because classical mythology and high fantasy are so boring, but there's this cool cartoon about colorful ponies, everyone loves it, let's add something like that to the game, which has always been based on mythology and high fantasy.

if this was transferred to, for example, Heroes 5, then this level 7 vampire would be the size of a cyclops... it's some kind of nonsense....

No, why? There was a witch on level six who was a small unit. A small unit on level seven is even cool. But vampires on level seven in the undead hierarchy just don't fit, they just don't. Dragons are far from them, but they have death knights, liches, why is the vampire better than them?

and I don't know why they're making so many dragons, and they're all so similar. There are enough castle dragons, and the bone dragon with 1k HP.

No, no, you can't have too many dragons, dragons are a classic, dragons are good. I have too many people in my social circle who love Heroes for its dragons, yes. And I love dragons too, what can I say.
The problem is with their design, that's what's bad.