Empire Total War had explosive mines, Shogun had the bamboo firing lines, rome had the burning balls of hay, and Attila had barricades. These little additions personally made defensive battles and sieges so much more fun, and I know it might not be 100% lore accurate it's fun to think about.
Dwarfs I think obviously get mines
Skaven suicide bomber rats.
Empire should definitely get barricades they can fire over.
Undead that should get a crypt pit that collapses and spawns a free zombie for every unit it killed.
orcs should get a rolling flaming dung ball.
Brettonia should get flaming tar they can set alight.
what are some of your ideas?