That is an interesting idea.
Were TFPs to do something like this a couple option toggles could be very useful.
a) simple 'Enable Overcome Zombies?' (or End Game or whatever fun term) (as mentioned already)
b) If A is Enabled, maintain total zed count or reduce as conquered?
-- for the 'never tough enough' crowd, instead of by taking out the animal shelter no more zombie bears/dogs, so lower overall 'tough' spawns, increase the spawns of the remaining ones. So if say a player left the Insane Asylum responsible for screamers until last, they might expect to see screamer hordes crashing even a low level house clearing.