This is all I can contribute (and bug finding) as I'm a player only.

Feel free to add your own suggestions:

Tactical terrain: Swamp tiles to limit your movement and defense, wooden areas to reduce damage from ranged attacks, arrows that can be stopped by obstacles that are in the line of sight, hills (height that give a +1 attack bonus to damage), catapults that will destroy fortified tiles and so on...

Traps: An option option to setup traps at the strategic and tactical level at a cost.

Treasure: when conquering a town you get: 0$ and 0 items. Can we gain a little loot please or my followers will abandon my rampaging urges!

Healing Champion's injuries: There must be a better way than to ruin my economy to buy an expensive potion to cure 1x condition. (infirmary building?)

Loot: It would be nice if we could get more loot when defeating enemy champions (items and or gold).

Thanks!