Not that it's a very broad spectrum of characters involved, and even less that might be involved in this idea, but...

Would there be any consideration to possibility of undead having a way to extend their lifespan after Becoming? I would expect this would have to be at an expense to power (be it sheer power loss, or a choice). In the case of liches, it seems as though it could be theoretically incorporated into something along the lines of the Ritual of Abominations. You'd lose your shot at a phylactery, which is a lot of the lich's power. Mummies...well, that I'm not really sure what might work offhand

Anyways, it's mostly a thought that if the undead is cool enough that either through direct Immortal intervention, or quest power, they could gain some means with which they could fight for extended life. I would expect it to be a losing battle no matter what, but it does seem as though it could be kinda cool to watch some great lich as the magic of the Becoming has begun to fail, and maybe tries to reinforce that spell's power to extend the unlife.