Some things cannot be carried easily or by just anyone but can be used, lots of weapons and heavy equipment, corpses, souls... not something easily carried away by a scout but something that should be represented and be able to be used.

So, how about battles with deaths greater than the number of survivors (just > or 2,3x greather than) drop additional fixed loot that can only be accessed by special units... mages can build a building to harvest corpses/souls and caravans/pioneers or a new "scavenger" can set up a route harvesting items until the battlefield is cleared (scavengers being able to clear most in one trip). Obviously this would mean that it would be easier for a defender to harvest the battlefield due to the low distance to nearest city which would make more sense as the locals come out to pick over the remains.

The extent of the placed resource would scale with the number of deaths for corpses and with the cost of equipment for scavenging... the resource generated could simply be 'battlefield' or one of a series of battlefields, tiny ones all the way up to massive fields full of pyres and piles of bodies. It's something I'd like just for easy necromancy visualisation. (add a necromancer to a battlefield and make it able to recruit undead... like the recent modding video)

Still, I'm sure most could be done with modding. But I'm not sure how easy it would be to make resource creation on unit death where unit's lost > victors with it creating a finite resource... hopefully easy enough