My thought is that zombies on scourge can "evolve" and have better equipment as time goes on through a wipe, but their equipment is never good enough to "overpower" a fresh-spawn.
My idea is, let's say,
Week 1 would have zombies with tier 1 armor and weapons, such as hemp clothes and the normal "bloody" masks and EZ shirts and such.
Week 2 would have some zombies start to wear wood armor and better weapons, like the spear.
Week 3 would have some zombies start to wear metal armor and better weapons, like the stone spear.
Final week (before wipe) would have some zombies start to wear heavy plate and use chainsaws.
These stronger "boss" zombies could offer better drops (gunpowder -> bean cans -> c4 as the weeks progress), so "zombie hunting" could still be engaging as the weeks progress and players become more geared up. The better drops from the stronger zombies could help people who joined later in the wipe catch up in terms of gear and stuff, when compared to the people who came in on wipe day.
Basically, the idea is to keep zombies engaging throughout the wipe cycle, without the zombies being overpowering to a new player, even on the last week. The bow that new people spawn with (should) be good enough for the occasional "boss" zombie. That's the idea, anyways. The picture shows the idea, but it's made in MS paint so take it with a grain of salt.
