The only issue I see with this is how you put them into play.
Do you add a specific zombie that only digs underground? If so, how deep do you have them dig, how many do you add in? How do you keep them from digging to bedrock and collapsing portions of the map, possibly under a base that is...
I would hate this with my play style, I scavenge for a few days and then settle normally on day 5 or 6, but I'd be really interested in this as a form of progression. It doesn't make much sense to me that you can make a minibike, but you can't make a backpack that can hold more than when you...
I could actually really see this in a server online, the towers could go by chunks(I believe thats the right word from what I know of the heat maps), as players group up and become established they could go to the towers and activate them for x amount of time to allow supply drops in the future