I realise this isn't a big idea but I wondered if this might be a possibility.
We have all by now picked up various animals along the way in our travels.
Could there perhaps be an achievement for gathering an army of just animals with the hero?
For example:
10 animals - Challenging
25 animals - Advanced
50 animals - Godly
100 animals - Epic
It;s just an idea really and would encourage a bit of thought when moving your army around to protect the numbers that you have. This wouldn't mean not building any regular units as you could move them to a separate town in order to gather your animal army in 1 town only