To give some importance to the picking up the gravestones of your own fallen Hearthlings, I think any enemy necromancer units should be able to come to a gravestone and raise your dead Hearthling as a skeleton or perhaps zombie.
Not only would this make it important to move the gravestone after a Hearthling dies, but you’ve also got to protect your cemetary! Imagine having a cemetary with a dozen gravestones and suddenly a necromancer makes it past your defenses and raises all of them simultaneously…
That is a very cool idea. Especially if you dress the zombie model in their latest outfit, or add their hair or something, so it actually looks like a zombie version of that Hearthling rather than a generic zombie.