Ideas and suggestions.

I suggest changing the cost of purchasing a hero: the higher the level and stats of the hero, the higher their price. This will speed up the start of the game and makes logical sense: a strong hero will require more gold for their hiring. I also suggest making various upgrades (+1 to attack/defense, stable, library, etc.) stackable, so you don't have to spend a lot of turns with main heroes, which would again speed up the game. Also, I suggest removing the penalty for cities based on their native lands (Tower for snow, Fortress for swamps, etc.). These are already not the strongest cities, and they are also penalized. + this will again speed up the game.