I want to get some new armies but I really don't want to have to mess with any new rules. Yes, I know that sounds sad. I was told Filth have some non standard rules so I'll be passing them up, are there any others that have non standard rules as well?
Every faction breaks the rules in some way. They are all relatively minor once you understand the base game. If you can implement a new event card or Summoner card text into your play, you should be able to easily implement the mechanics introduced in both the Swamp Orcs and Filth factions.
Swamp Orcs is also included in the Master Set which I think gives you the best bang for your buck!