Problems as mentioned before is the slow weapon swap, long reach of the zombies and long wind up of monkey bomb. They could change all these things and still the game will have one flaw that can not be changed and thats having 2 or 3 zombies working as a team to beat the human. The zombies camp round the corner and rush the human at the same time, and one will get the human unless they are utterly useless even with a faster weapon swap. I only got my achievements because I was with noobs and some good timing and insta kill and monkey bomb arriving at the right time.
Would rather see a game mode that includes 2 humans against 6 or more player controlled zombies with open maps like tranzit or die rise. The 6 player controlled zombies are alongside computer controlled zombies, and the 2 human players can not tell which is player or computer controlled. That way the humans can not do the usual kiting and the player controlled zombies can use some tactics with the cover of computer zombies. The game mostly plays like the traditional zombies, but maybe without the heavy weapons like ray gun, etc.