Game starts with mostly normal zeds, and static roaming hordes and cities filled with zombies.
As time goes by, an increasing number of zeds begin to transform into boomers, zombie masters, zombie necromancers, brutes, etc.
Further, various twisted mutants climbing up out of the sewers after a certain amount of time goes by. Sound should go further. Even 1 unit of noise will slowly, and in a very gentle matter, “tug” all surrounding zombie legions toward you.
This would have the side effect of forcing you to fight increasingly huge hordes if you try to stay in one area for a long period of time, and that difficulty will steadily go up as time goes by.
I would also like to see occasions of zeds swarming over non-human (triffids?) populations and duking it out. The zeds seem like they should generally win with long distance shock troops and the ability to re-raise their own numbers, but their opponents should have the advantage of being able to replicate their numbers instead of just “refilling” their army with re-raised zeds.
With this in mind, “fresh” zeds should always be normal, regardless of what they were before.