Just a small UI suggestion - please implement a tabbed spellbook for strategic casting.
There's a fair bit of space along the edges of the spellbook in the strategic UI - use that space! I'd suggest putting tabs for the spell element along the left side of the spell book - so a tab for Fire spells, a tab for Earth spells, and so on. Along the top of the spellbook I'd put tabs for spell effect type. For example, overland spells, city spells, unit spells. Take a look at the way spellbooks are implemented in other games like MoM or the HoMM series for examples.
It might not be bad to also have the spellbook available in combat, while retaining the current actionbar for frequently used abilities. If this is too much work to implement at one go, I'd suggest working out the UI for the strategic spells first. Then just add a button into the combat UI that opens the spellbook. In the combat spellbook I'd suggest tabs for classes of effects - direct damage spells, buffs, debuffs come to mind.
The game is progressing along, there remains a lot of tuning work to be done. The game will need quite a few balance passes to get things challenging and engaging. Tone down the direct damage output of champions while buffing their supportive functions for one. Champs should, in my opinion, have high survivability and should serve as force multipliers - but they shouldn't be the entire military in a small group of heroes. Also, it takes too long to produce units in the early game relative to the rate at which your sovereign and starting champs gain power. Because trained troops are always so woefully behind, they remain useless.
We have a set of conditions that lead to heroes becoming too powerful too fast. Equipment drops are too good too fast, and have too great an effect on a champs output early game. Either the leveling has to be slowed, or level requirements on gear needs to be significantly boosted, or perhaps some other option.
It's too easy to steamroll opponents. Taking an enemies capital should be as challenging and meaningful as taking down one of the world bosses in the game. Put large static defenses in the capital cities, as a baseline! Currently, the winning strategy is too easy - farm experience and loot from world locations and monsters, then steamroll one opponent after another. Perhaps lower equipment drops and buff gold drops instead as one part of the loot adjustments.
The balance problems are pretty multifaceted, I think keep balancing things out and refine it, the game will get there.