Is it possible to make procedurally generated monsters? I think zombied or non-zombie NPC-like mutants using the player-type mutation tables would be interesting. Make the post threshold ones more rare or distinct and maybe tie them to specific locations like labs, but considering the society in the backround lore, you’d probably have a few random mutants in towns and forests and sewers and whatnot.
Are there alligators in the sewers? If not, there should be. Possibly zombie alligators.
My only other thought is a vauge desire for otherworldly enemies that are more dangerous then the robots we have already. It makes you wonder how civilization collapsed when a single properly programmed tankbot could/can destroy any number of any non-tankbot entity around.
Oh, that’s an idea, how about an extradimensional something that uses an EMP attack that drains any weilded/activated/in use energy weapons, bionic power, vehicle batteries, ect? It’d explain all the cars with dead batteries everywhere in New England. Well, that and the EMP from the nuclear bombs, I guess.