Its certainly better to implement the evolution system itself for the existing zombies first and then perhaps think about adding new ones.
Who they will be transforming to then? We’ve got a lot of zombie of different kinds with no link to each other. The longest chain of evolution I can imagine is zombie->tough zombie->brute zombie->hulk zombie->Thriller. It would seem just strange to have a zombie transformed to, say, zombie cop. So yeah, despite having lots of enemies, they usually don’t have links to others.
Another way to make evolution (I suggested it a long time ago) is to not create new zombies, but strengthen existing. More HP, higher skill, higher speed and stuff. It might be done easily, but it’s not really that interesting. Without some, um, randomness evolution is not going to be interesting because after a while any player would learn the way it works. So yeah, without some randomness (of perhaps random enemy generator) this feature is doomed to be boring after a while.
Also it’s worth saying that even with zombie-lution zombies would not pose a threat for advanced character. Any player would know how to deal with some enemy if it’s not possible to just go and slash their throat with almighty katana, so that’s why random zombie generator would be far more better. If player have no idea how to deal with enemy - it becomes interesting to fight with this new kind and probably die the stupidest way. Just imagine random made a player a present - zombie who looks just like standard green Z, but is able to spit fire as tank drone. Now imagine how !!FUN!! things will be if the player is melee-based. That fits nicely with this idea (http://smf.cataclysmdda.com/index.php?topic=9734.0), but in other way - you learn about some kind of zombie as you see it using its abilities.
So, that being said, I consider this the best way to implement evolution.
Tough zombies aren’t an evolution of ordinary zombies. They are just zombies of people who were athletic/muscular.
I imagine the “zombie evolution” won’t make them transform, but instead tack on new qualities. So a “fat zombie” would become “mutated fat zombie” with increased abilities, but there wouldn’t be an explicit definition of “mutated fat zombie” in the jsons, just the “mutated” modifier.
See this for inspiration:
http://www.diablowiki.net/Boss_Modifiers
http://diablo2.diablowiki.net/Monster_modifier
We’d most likely make a bunch of intermediate types to extend the chain a bit, or just rely on the concentration of more advanced zombie types increasing to make things interesting.
Evolution isn’t a system for making things interesting, it’s a system for difficulty management. You’d start with a bunch of vanilla zombies, but over time more and more of them are going to convert to more dangerous types. What it would however allow us to do is add zombies to the top end of the scale without having to try and balance them for characters that are just starting out.
As above, evolution would hive us room to introduce far more difficult monster types without having to worry about making early game impossible.
I’m not opposed to having random elements in there, especially if they crop up somewhat gradually.
You'd start with a bunch of vanilla zombies, but over time more and more of them are going to convert to more dangerous types.It smells like dynamic spawn. In beginning you get puny zombie children, in end tankbots nuke you in both meanings of the word.
What it would however allow us to do is add zombies to the top end of the scale without having to try and balance them for characters that are just starting out.It smells like dynamic spawn again. I think the best way to do this is make something like Risk of Rain's time2difficulty system. Player should be forced to become stronger.
I think this would be awesome. In fact I would argue this could be one of the coolest things to add for re-playability. A lot of the suggestions I see on this forum are for tweaks that can be made for balance or realism but this is huge. This has the potential to make playing the same character continue to be challenging even after many in game days of playing. Or at least that’s my opinion. Other people can disagree.
I’m so glad this is being brought up/re brought up. 
Kevin, you just made my day!