Yes, that was part of the original plan. I've been working on the buy/upgrade system, but also has to update the way you harvest the zombies. The better the grave is, the stronger the zombie (like a super zombie, skeletons, berserker, etc).
Not sure if I'll have the time to do so. I'm already working on two commercial games, so this is kind of a side project. The ideas of course keep coming to my mind and yours are welcome too.