So ARPG and Zombies would be insanely addictive if done right. A big part of ARPG games is builds, classes, and character development. COD lacks this. Itemization is a huge deal as well, and there isnt any of that in game at all.
I think COD needs to catch up to 10 year old indy games zombie horde games first, before it tries to break the mold and do something cool.
Simple stuff like not having a safe house goal, or a safe place to upgrade, would go a long way. Classes having an effect on how you play would go a long way especially if done with support to others ben