Me and Spartan224 had a discussion on what the new zombies were gonna be in the extension and the ones that are planned to be included were:
The Crawler - A zombie who only crawls on all fours. It is also faster than a normal zombie. If it jumps into a survivor, it can make them lose the current weapon their holding. This zombie has 180 HP.
Hazmat Zombie - A zombie who wears a hazmat suit and is completely immune to fire damage. When killed, it has a 60% chance of dropping a bile jar. Other than that, the zombie basically is the same as the regular zombie.
The Leaker - A heavily radiated zombie that is pretty similar to the Boomer, except it does not have a vomit attack. When killed, it explodes and leaves behind toxic gas as well as a puddle of acid that resembles the Spitter's acid. It will probably have 150 HP.
The Trasher - A fat, slow zombie that is also pretty similar to the Boomer. It is pretty slow, so it camps behind objects in an attempt to ambush survivors. If the survivors get close, the Trasher reveals itself and vomits a goo-like substance onto the survivors that slows them down considerably due to the crushing weight. When killed, it leaves behind a small puddle of goo that immobilizes survivors for 10 seconds if they walk on it. Might have 215 HP.
The Stalker - A zombie from Roblox's R2DA. It is a tall, grey zombie that cannot damage the survivors in any way. Instead, it snatches survivors in its arms and takes them into a horde to have them finish him/her off. The Stalker can also generate a force-field around itself, rendering it invulnerable to enemy attacks. It can also turn invisible if it stays still for long enough, but will soon turn visible again if it makes a sudden movement. A quiet whispering sound can also be heard when it is near. It has 250 HP.
However, all of these zombies are just mere ideas and may or may not actually be added in.