Even in the sixth installment, each race could be made much more unique.

It’s possible. But the code is too capricious for the developers. It accepts new things with great difficulty. It was designed in such a way that changes are difficult to implement.

It would be much more interesting.

At least, the spellbook was well thought out in the second part of Heroes. In the third, everything was divided into elements, and that's where it could have stopped. The spellbook doesn't need changes. Except for some cosmetic adjustments. And you could remove a few spells and replace them with some new ones.
In general, this hero upgrade system is convenient on one hand, but on the other hand, it only causes a headache.