So. No concrete examples here... But

A troop that brings back dead troops at the expense of others in your army OR at the expense of itself, bringing back a nerfed version of the troop.

How would it work? God knows how.

How would it invent new strategies? Well.. Who doesn't want an undead pekka?

How to defend against? There would be a weakness of some kind. Possibly only certain amounts per battle.

Leave your ideas, as I say, it's just a thought I had, nothing concrete