Just a quick reply as im texting fron work....
I would recommend basing them as your slaves army, however to make them stand out more or look less plain in the warcry game, i would suggest adding some extra debri (such as ruins, or bones/skulls...erbody loves skulls) that way they both stand out on the warcry table and fit with your slaves army.
I am personally basing my untamed beasts with wintery snow/ice bases. I always interpret my own lore/make my own which adds extra depth to an army, for example YES untamed beasts come from a certain realm but hell, there are beasts in every realm so who is to say thay cant come from, or have migrated to other realms. Games workshop is amazing for its rich lore and openness for its players to create their own story. For example instead of my gloomspite being in dank caves with mushrooms everywhere, i based them to look like they live in a duardin ruin.
The biggest challenge for me with my bases is making them realistic...i live in Australia, we are currently in our coldest month of winter and it was 27°C (80°F) yesterday. Im wearing shorts and a t-shirt and trying to avoid the sun haha ive never seen snow in my life!