Well, it would stand to reason that these wasteland areas were caused by a tactical nuclear weapon targetting main cities to help stem the tide of the infection early which, obviously, failed leaving areas that have been scorched earth destroyed and irradiated. The outer perimeter would be burned due to the extreme heat caused by the explosion. This would put some irradiated cities in the middle of our maps which would require radiation suits to be able to naviage and explore. This concept would accomplish a couple things: 1) actually give a significant use for radiation armor other than scrapping for military fiber to use for making better military armor, and 2) provide some realistic canon as to what happend in the aftermath of the outbreak and the lasting consequences. I remember back in A15 when hub cities were still a thing, the hub city would usually be huge, but it would be pretty much wasteland at the same time, indicating that it had been hit with something massive.
As far as I can tell, a 8km X 8km map typically has about 10-15 cities of varying size from small hamlets to major metro areas. With a little tweaking to the seeding logic, you could easily make it to where your smaller towns were generally safer from the radiation since it would be unlikely they were targetted while your larger cities have a chance to be nearly in complete destruction and irradiated. So if there were 5 or 6 large cities on the map, then maybe half of them would be wasteland cities surrounded by burnt forest. This would create a challenge for those wanting to explore and be consistent with real life ideas of how an outbreak might be handled.