[Gameplay] specialize magic by book (fire magic aggressive, water magic defensive etc...)
As it is now one doesn't really need more than one book of magic, since fire spell can bring damage of all sorts, but they usually have an equivalent in earth magic or air. My suggetsion is that by choosing fire magic one should choose between a large number f aggressive spells, but lack defensive ones. Water magic could bring healing as a spell, but be weaker in terms of offensive spells. Earth could bring spells that affect the strategic map and air should add speed to units (please be careful with teleporting spells, because they tend to spoil the game since entire stacks could just appear out of nowhere thus defeatinhg any attempt to carry on intelligent tactics. Movement is always the most important element in warfare.)
As it is now players don't even bother researching 90% of spells since the lightning that you get at the beginning is already a very decent one so why having both fire ball and chain lighting?
(PS adding immunities/resistance to specific schools of magic would at least give some sense to variety of spells)