I think it would be good to have 7 schools of magic, like in Heroes 6 and 7.
Original
Light
Darkness
Earth
Water
Fire
Air
Then there would be variety, and the schools of magic would almost certainly not be repeated.

Well, I already suggested this, and after a heated discussion, the idea was successfully thrown out – there are many problems: where will we get so many magic skills, and there aren't that many spells to distribute them among seven skills from four – although it would be interesting, you could also use racial skills and make them available not for one faction, but for several (for example, rework irresistible magic into fire magic and make it available to dwarves, demons, warlocks, mages, and humans)

But this is tedious, and the question is also how multiplayer will react to this, as well as single-player maps and campaigns with scripted events, etc. – I don't think it's worth going to extremes, unless, like in Heroes 3, we make 4 schools: Water, Fire, Earth, Air, but specifically light and darkness, as schools of magic, and not something amorphous, are very characteristic of the Ashan universe...

And so, what was I talking about...
:D
How will we distribute the schools?

darkness-chaos:-----------inferno---------dungeon-----
light-summoning:----------humans---------sylvan--
chaos-summoning:--------academy---------------------
darkness-summoning---------necropolis--------------------
chaos-light:--------fortress-----------------

for dwarves, replace summoning on the path to Supreme Unit with machinery, and the problem is solved