I too would like to see a greater deal of diversity amongst the zombies to make things a little more interesting, but like with all things, the devil is in the details. Its important for me to underscore that in the "Commonly Suggested Suggestions," thread the development team have gone ahead and confirmed they will not be adding:
All right so what does this mean? Well it doesn't necessarily discount the idea of your fat zombies, and perhaps not your strong zombies, provided these traits are incumbent on how the person was prior to undergoing reanimation rather than it being by virtue of the zombie virus itself giving them these abilities. I could see this coming if/when the developers choose to add more body types to reflect your weight and nutritional status in the game (you'd see muscular characters, fat characters, skinny characters etc.) and their respective zombie forms would thereby possess these traits for a time at least, before the process of decomposition would likely eliminate these depending on the zombie lore settings you're using. Not much is known about the developers' plans regarding said body types being implemented, near as I can tell, however searching through the Discord I see references to them considering some "mesh morphing," method to accomplish it.
Also regarding your other suggestion
A system akin to this is already in place, however I don't know that it matches what you're looking for. Nevertheless I think its pretty good, don't you think?
In any case, some of your more ambitious ideas regarding the evolution seem to clash with the developers' view for the game.