We could also introduce two new resources: iron and ebonite.

I've also thought about that. Iron could be a special resource for Haven. Bones for the Necromancers. Wood for the Elves. )

So:

1. Non-linear battlefields. That is, not just a square with troops fighting on it. Try more complex shapes. Make it impossible to shoot through certain obstacles. For example, trees. And make some locations (like a swamp) have a higher concentration of trees. Then archers will only be able to shoot through the areas where there are no trees. This will enhance the tactical aspect.

All sorts of passive obstacles. They don't block the path, but they slow down/deal damage. In the same swamp... hmm, a swamp. It slows down a creature moving through it by 1 square.

Underground buildings (battlefields) of various geometric shapes, again to enhance the tactical component. The shapes can be very diverse - from "+" to rectangles (long, narrow corridors).

2. I have this idea: when 100,000 melee soldiers attack, only the first ranks actually hit the enemy. So, I suggest introducing a kind of "damage limit" - damage is dealt only by as many creatures as the defenders have. That is, 200 centaurs attack 100 goblins, and the damage is dealt only by a hundred centaurs. The system is conditional, but I think the main idea is clear. Now, necromancers with their hordes of skeletons can be contained more effectively.

This system works especially well in conjunction with the fact that global time and arena time will be linked.