The whole point of freeplay from our point of view is that it's a very efficient mode for development. We can take the things from the campaign and re purpose them - we don't have to get Matt to make entirely new models, textures, and animations. We also don't have to re-do an entire behavior tree for the creature.
If we started asking Matt to make unique creatures, we delay the campaign. It's as simple as that.
I believe the current thinking is the uber creatures will be visually different in some way, but they'll use the model and animations from their base creature.