This is all just my opinion, so take it with a grain of salt.
I think one way to particularly make this game great is to make sure there are a great many spells that have non direct-damage applications. Let me explain.
Typically, in the fantasy milieu, you will have the game designer saying stuff like: “There’s a HUNDRED different spells!” However, those spells typically look like this:
Fire bolt, Ice bolt, lightning bolt
fire blast, ice blast, lightning blast
fire storm, blizzard, lightning storm
There may be minor differences in their behavior but they are largely a palette swap. Nothing interesting here. Nothing to get your creative juices flowing and make you think about the game and the choices you make. Frequently, you choose between them for aesthetic reasons as much as anything else.
I think the key to making the spell system interesting is creative asymmetry between the spell schools. Part of what made MoM great was how they did this- have some schools be more about summoning and empowering, others about controlling (countermagic and the like) others about maneuverability. Have spells that deform the terrain on both battle and strategic maps, spells that affect commerce and research, spells that affect diplomacy. Go crazy with magic- “Pocket Dimension”, a spell that allows you to build a fortress redoubt in another dimension, only reachable by enemies that research the same spell OR that find a “key” item that is randomly spawned in a lair somewhere in the world when the spell is cast. Make a spell that tears the veil between worlds apart and randomly starts spawning units taken from elsewhere in the game’s metaverse- neutral units that attack all players! Have a spell that drains the casting wizard’s magic into the force of Steel, giving them special technological units and a huge production bonus at the cost of some or all of their spellcasting abilities! You could have a spell that, once researched, allows you to blast a spell out of an opponent’s spell book, at the cost of having to re-research that spell to cast it again. Go nuts!
I know what you’re gonna say: using the mod tools you, Mark L, can make all of this happen! Sure I can. I can make it happen in a chaotic, unbalanced fashion that always “feels” like cheating when played against the AI. The thing is, I have faith in you guys- I have purchased every major release you’ve made since GalCiv 1, so I know a thing or two about what quality game-makers you are. If you craft all of this* into the world, I will be able to play with the knowledge that it is well tuned and balanced, and it will feel “more fair” when I use it.
Anyway, here’s the tl:dr version: Creative asymmetry is the way to go! :)
Good luck on your game, and I look forward to being first in line to purchase it.
*these are just random examples off the top of my head, o’course