Built-in autosave feature (for the TE version).

The most balanced approach would be to have Mass Slow and Mass Haste as separate spells at level 4, similar to Heroes II.

In Heroes II, mass spells are indeed separate, but there are no schools of magic. And by the way, Mass Slow is valued more than Mass Haste.
Mass Slow costs 12 mana and is on the 4th tier, while Mass Haste costs 9 mana and is on the 3rd tier, just like Mass Bless.
Here, the question is whether to not interfere with the magic system from Shades of Darkness, or to change not only Slow if we do.

Heh... maybe for now, all warriors should be prohibited from leveling up Earth and Air magic to Expert? And Mages will rise, and the schools of Water and Fire will also rise :D ... and then come up with something more serious...