A truly F*cked up and disturbing idea…and I like it! lol
But if I may. How about the borrowing the Triffid system. Instead of fungus growing and spreading. There is a new creature that rips both zombies and people apart. Eats them. Then lays eggs similar to Aliens queen. The difference though is there are “minion” creatures or drones that mutate the land and plants. So all the body parts form into one large living mass. The ground is like a pulsating fleshy floor of tendrils. The trees are arms and legs and heads of people.
Hang on I’ll go grab some reference material.
Edit:
Ok so much for that crap. I wanted some footage, but I couldn’t find anything easy enough to post here. In short go check out youtube for these references:
Silent Hill film / Janitor scene in which the twisted dead guy touches the wall and the wall turns into fleshy “oh noes!” kinda material. Kinda the way Triffids morph the land. Perhaps also watch The War of the Worlds(Tom Cruise) to see the aliens transforming people into red people plants.