Nice Undead idea, but i don't like that body grabbing thing. Maybe Undead can have a promo with chance to convert unit to their cause after succesful battle. But converted unit will be changed to undead of type according to his unitclass(melee->skelies; archer->skelie archer; recon->whaiths; arcane->necromancers). Also it can be scalable through level of unitclass:warrior->skelie; axeman-> zombie; champion-> diseased corpse. Maybe even some promos could pass aswell. Also some mechanic after seizing a city can be fun, like if Undead horde capture a city with mage guild ->chance of necromancer\lich will created. Similar things can be applied to other buildings as well.