I'm addicted to this so here comes the time to add gratuitous suggestions, hoping that my fix becomes even better in the future:
– Possibility to cancel a purchase or upgrade, once per game – or twice, if no buying or upgrading has been done since; makes it less penalising if it's just to correct an inattention mistake, but make it decisive and limited if the player wants to use it to reorient their strategy long after the purchase or upgrade in question.
• Alternatively, making this a common pool of "hexes" along with spying, that way choosing to use either has to be strategic. Once they're depleted, spying is still possible at the cost of 2 coins but cancelling is no longer possible.
– Possibility to "Research" a unit, that is, paying one coin during the prep phase to increase the odds of having it pop up in the roster next preparation phase. This would let the player use spare coins at the end of a turn with a bit more control than just re-rolling.
• Such odds increases could be proportionately leveled to the current size of the roster – or not, if you deem it too powerful for higher tier units.
• "Research" could be a 3-items alternate roster, where units available for research are randomised each prep phase ? That way the player doesn't get to research any one unit. But maybe a second randomised roster would be perfectly redundant.
– "Advanced research" would be paying 2 coins to be sure a given unit appears – powerful, but perhaps still balanced as the unit effectively costs 5 coins to obtain, over the course of two turns.
If Mausoleums make this too easy to use, then Advanced Research could cost 3 coins, bringing the unit's total cost to 6. Expensive, but that would be the price of cheating rolls and encourage the player to trust their luck a little.
– Something that's meant to decreases enemy offensive power, or "stun" an enemy unit?
– A building that lets the player swap any 2 units on the fly when battle has begun ? Once per level of the building. As for theme, it could be either:
• An observation tower (with a spotter for strategy)
• A standard bearer (they were used for armies' coordination)
• An underground rail network, dug under the graveyard. Because I believe it would be really funny that undead minions would reorganise their formation during combat by… taking the metro.
– Other "campaigns" that would pit the witch against different bosses with different mechanics, such as :
• one who shows up after a fixed amount of turns, and whose power is instead proportional to the number/level of units the player has, forcing to adopt different optimisation strategies.
• "multiple" bosses who would have to be defeated all at the same time; each could require a different lane optimisation.
• "Sequential" bosses who'd be fought one after the other, with a progressive increase in power (Granted, that's far less original than the King's "grows stronger with time")
• A Rival Necromancer using "mirror" units and spells?
– Different types of protection, like a light armour that only stops 1 damage – useful against retaliation attacks, but that's it; maybe make it only work against such projectiles; or one that halves all damage received (rounded at the player's disadvantage), starting at 4. That way it's more efficient against very high damage, but no better or even worse than a shield against anything lower than 5.
– More RATS! (I love rats but no pressure)
Am very curious to see what you will come up with in the future, especially the "alternate decks".