Harmony - Troop Symmetry

Oh, come on, let's not make everything like it was in the third game – that's absurd. Earth is something large, unwieldy, sturdy, robust, indestructible, slow, and powerful, and most likely not flying. Besides, the argument about lack of originality isn't valid, because just as you said, "walking dragons aren't original – the Dwarves have similar ones," you can say the same about a flying Air Elemental dragon – the Elves have similar ones
In terms of tactics, the Unicorn has been replaced by a Phoenix – meaning the ratio of infantry to flying units in the elite troops hasn't changed.

Yes, the Elves also have flying dragons, but that makes sense – flying dragons are a classic, walking ones are an exception. Exceptions shouldn't become the rule, appearing immediately in two factions.

Besides, there's a fundamental difference between light and dark dragons – it's initiative and power. The former have initiative, while the latter are more powerful.

Oh, and if a level 5(!) Unicorn is replaced by a Phoenix, then replacing a flying dragon with an infantry unit is the lesser of two evils, of course. :)

Why not keep the classic Unicorns and replace the dragon with a Phoenix? Or was the Phoenix specifically designed as a flying unit to replace the Unicorn? If so, it would have been better to try making Pegasi, in my opinion.

And "earth" doesn't really fit with a rushing faction; you'd have to completely rework the entire faction. In terms of gameplay, I'd say that the Elves worship the dragon of air, not earth. And it's in this context that the dragons from Heroes 7 don't fit here. But if you really want to see them, then you could consider them as a replacement for the Ents.