If you continue to see battles as mage duels, then
at least give players the ability to learn transformable spells, or consistently grant teleport and damage spells on the final transformation. (Perhaps sell specific spells at a corresponding price.)
Consider how to make the growth from castles more powerful compared to the stacks that are gathered from the map, which are essentially free. Remove top-tier relics from being found with weak creatures like Trogolodites. Relics like the Crown of the Underworld, the Crown of the Vampire, and the Hat of the 5th Level are also essentially free.
I also have a suggestion regarding summoning magic: rework the monsters into upgraded versions, reducing their numbers. Currently, there's no alternative to Earth Elementals (and in general, it's the top magic in the late game).
I've always been a fan of Heroes games, but now, a one-button Necropolis strategy seems more appealing to me. That's how I see it.