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Well, for example, you could make it so that monsters don't have just 2 upgrade levels, but many, like in Master of Magic – novices, regulars, veterans, elites, etc., so that they also gain experience.
Added 20 minutes later
You could also create a new race – werebeasts. That is, all sorts of ratlings, wolfmen, bear people, etc. That is, they are like living people (not undead). But when they go into battle, they do something like a gatling gun – they wait for a while and transform into the corresponding animals, which are very strong and ferocious, and their rage increases with each turn, but if they fight in animal form, they become more and more savage with each turn and with increasing probability they will lose control, that is, they will attack their own, just run around the battlefield, etc. To somehow control them, you need to upgrade the corresponding abilities or cast the corresponding spells.
But they can also fight just as people, but in this case, they are not very strong.
The hero has an ability that is something like necromancy – lycanthropy, that is, you can turn people or elves or mages into werebeasts.
Added 6 minutes later
You can also add to morale that it increases or decreases depending on success on the battlefield. So that there is not only courage, but also cowardice, that is, some nasty cowards simply flee from the tactical map to the reserve or just nowhere. Or surrender to the obvious winner, so that after the battle they end up in his squad.
Heroes can be captured, and the experience for them is not simply 1500 for all, but depending on their level and upgraded abilities. That is, defeating a strong hero is always honorable, and not just a novice of level 1-2.
You can also make it so that, depending on the player's alignment, there are such possibilities – generously release the prisoner (and you will get some kind of bonus for it), but the opponent may immediately hire him in the tavern and it starts all over again. And an evil player, on the contrary, can sacrifice the prisoner, in which case he will disappear forever from this map and all taverns, and the one who sacrificed him will also get something.
You can introduce the concept of city population, as in Civilization or Age of Wonders, and it would also grow from turn to turn, and the city's parameters would improve, and there would be more monsters, and more money, and more resources. And if the city is captured, the enemy can massacre it, destroy it, or completely eliminate it.
Maybe it's worth making something like Settlers and letting cities multiply.
Another option is to add oceanic races.
There are many other things that can be invented!