I must say I am mildly disappointed (not by the art, which I liked), possibly because I had very high expectations. Here my comments.
The things I liked
1) There are tactical spells, strategic spells, universal spells, city spells, enchantments. This is all very good! – except that World spells are mostly missing L
2) One cannot have both Life and Death magic. This makes sense and adds an interesting specificity to the choice between Kingdoms and Empires
3) I appreciated the nice pdf document!
4) Quest spells are a nice idea – within certain limits, see point 3 below
5) Combined-school spells are a very cool idea
6) Some spells are quite innovative: growth and shrink for example, I like them!
Something that could be improved
1) More spells: A game like HoMM V could live with just 2/3 spells per magic realm per level only because the game focuses mainly on tactical spells. In a MoM-like game, as developers have rightfully done, there are tactical, strategic, enchantment, city-spells, so a much higher number of spells is required;
2) Broader effect spells: Here again MoM is a good example (sorry folks, but it's true). Every realm had several World spells, and they were among the nicest. This was also a very unique way that magic could influence the world. Some suggestions: make a spell that increases the regenerative power of one’s troops all over the world (Blessing of Life?). Make one that decreases if for enemy troops (Festering wounds?). Make a spell that decreases the movement of all enemy troops in one’s territory (Dark Fatigue, Guarding Winter…). Make a spell that reveals all shards on the map that are within a certain distance of your territory, and an improved version that shows their type. Please, do make world spells an important piece of spell choices. Also, please make tactical-global spells, i.e. spells that affect all units within a tactical combat. Some examples from MoM: Prayer, Dark Prayer, Call lightning, Blur, Flame strike, Warp Reality, Metal Fires.
3) Randomness in spells is nice, but it should not be overdone imo. I like the fact that every game will be somewhat unique, but I personally like to be able to plan ahead my game to a certain extent. I know you want to avoid that people plan everything ahead and end up always playing in the same way, but using random events and spells is only one way to achieve this, the other being to design the game – and the spell system in particular – so that there are many different combinations of strategies that one can choose, all of them interesting, different, and powerful - so that a player wants to try them all. Bottom line: when choosing how to specialize in magic, I think one should have a pretty good idea of what one is likely to get – beyond this, some randomness is nice to have.
4) minor point: I believe champion should be able to cast spells only within a given range. I can't imagine a champion casting a Tornado on the other side of the map, while a Sovreign sitting in the capital... well, it's a different story
Another idea for a spell: "Curse Shard" -> can be cast on shards owned by opponents and reduces the amount of mana that the shard generates
All this said, I am sure it is a very challenging task to create a spell system that fulfills all our expectations and is a balanced part of the game. So, good luck! I’d be happy to contribute with suggestions.