I was thinking some about the the whole champion spawn thing and it occured to me that in a lot of ways its actually fairly similar to our invasion spawner, which lead to this idea.
Suppose there were 10 or so locations in the wilderness (preferably in areas that are not really used much now) where the fabric between worlds is weak and its possible for an invading force to come thruough and expand into Brittania.
Functionally it would go something like this, when the invasion spawner runs there is a small chance, 5% or something, that an invading force has been able to make use onf one of these gateways and establish a foothold, gradually expanding in a circle (well square I guess) around the gateway point until they are found and beaten back. Pehaps something along the lines of the Ackadia dungeon spawner could be used for this to give a good range of possible mobs. It probably wouldnt actually be possible to have them actually threaten a town, well unless the invasion circle got really, really big.