What is this Purge you all talk about? 
Never had one on my private server, would be happy it would work as intended first.
As for world events it would be nice to see something along the lines of this suggestion:
Reverse Purge
Above could be dependend on how frequent you raid a certain faction in order to recruit thralls.
In my case, since I raid Asgard that much, since my base is close, it would be fun they might fortify their city further over time and maybe gather some friends in order to “Purge” me and my base. Same goes for wildlife, where I would not put the same thresholds for triggering it.
In my opinion this would make the Purge more related to your actions and you would see an impact on the game (fortification) over time.