I'm glad they never went forward with the behemoth.
It would be nice if the zombies/enemies are kept at the level they're at now. I think the Demolisher is a good example of beefing things up without going too far. Nearly all other zombie games are filled with these big claw-faced or tentacle creatures, which kills the zombie vibe.
If TFP needs a little inspiration when it comes to making more boss-like zombies, I suggest taking a look at the movie Return of the Living Dead 3, notably the Riverman. It would also be very appropriate with where bio-tech is in real life at the moment.
With horror, as far as I'm (and many others are) concerned, the more grounded something is, the more immersive and impactful it will be. That's what makes zombies great. They're simply reanimated dead people. Way more believable than big giant creatures.