Also, not just Deamons, but Beastmen and monsters(to represent the mutations caused by the the influence of magic) Also, I think even Chaos civ's shouldn't be safe from these mauraders-I mean they're Chaos, they battle each other too.
But since none of those units (except a coupla monsters

) are made yet, go for a more general approach. Have various factors (religion civic, Chaos religion spread, number of warpstone mines, I'll add a few buildings later as well, like Wizard's Lab or something) increase the chances of a spawning of a random unit or group of units inside of a civ's boundries.
Could you also try and make it modifiable through an ini file? (Look at Lopez's works) Just to make it easier to change, for example, the list of units who can be randomly spawned, some of the groups that might spawn, buildings and other influences.
The more I think about this, the cooler it gets...