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I always dreamed of a map like in Total War, and a separate model for each peasant, but still with turn-based gameplay. And then I found out that Age of Wonders exists...
On the topic: using NCF files and combat scripts, you can simulate something in the spirit of what you're talking about, at least with one hero. You can make the hero's model invisible and give him a creature in the amount of 1, which visibly acts as an imitation of that hero, becomes stronger along with the hero by replacing it with another NCF creature, and if it dies, the hero automatically loses.
I once brainstormed about such an RPG implementation as a thought experiment, but I haven't encountered it (although it might be on fan-made maps). An obvious drawback here is that such a hero will move twice, first casting magic as the hero himself, and then moving around the battlefield and attacking as a creature.
In general, it turns out to be a rather clumsy and time-consuming solution, and it's quite specific (it only makes sense within a separate map or campaign). To make it work properly and without being a kludge, you need to modify the engine (as usual – monkey-jpg).