I just dont think a digging z or magical digging z is that answer. Besides MM not liking holes everywhere due to a digging z also in which he stated in one of his videos. There are lots of other ideas that are better.
Madmole has also said the zombies are and always will be the main antagonists. So I think it's the most sensible choice for zombies to be part of the answer. You don't want them to dig and leave tunnels, dig and
not leave tunnels, or spawn up close to your base, which would seem to be the only options to get zombies to your underground base. So we're at an impasse - we simply want different things.
In every movie the zs become a non threat so to speak. Human race always wins.
While movies can inspire gameplay, there are also differences. Movies have an ending, and this game does not, at least for now. The survivor(s) winning the game in a few hours like in a movie would be one play style, but not the only one. Consider Royal Deluxe's play style: he mentioned he plays 'dead is dead'. I'm sure he finds it every bit as thrilling and compelling as someone role playing that they're the main character in a zombie movie.
The game needs challenges for sure but digging z isn't it. You might get 2 min of enjoyment challenge but after that hmmm back to not being challenged again.
Why. Not because tfp failed. But because the gamer smarted the game. Countered and neutralised all threats.
Again it don't matter if TFP add in the most challenging z ever. It will always be other players (PvP which I hate) That you can never counter for because that is the difference between alive and dead.
The players will come up with a way to counter everything the game can throw at them! Well, no, that's not the case for every game. It's possible to design a game where the player can't counter everything, and you don't have to brutalize the player with ridiculous rules to do it, either: I refer you again to rock-paper-scissors. Too simple? Okay then, chess against Deep Blue.
Me, I derive more satisfaction from winning through sheer effort than by finding a way around the challenge the designer intended for me. Sure, I feel clever coming up with the latter the first time, but after that the victory feels hollow. I've discovered how the game is broken, and now I can't put that genie back in the bottle. That's my "back to not being challenged again" moment: I can only have a balanced, challenging game after that by hamstringing myself, pretending the game isn't the way it is.