I would love to see the equivalent of flying dragons implemented via simple BIRDS. For a mouse, anything from a owl to a hawk or even a crow could be pretty lethal, and having to watch the skies and use arrows/spells/sling bullets against a swooping bird would be pretty tricky yet fascinating. Using the DX11 architecture, you could even utilize the shadows of overflying birds as an early warning...

Serpent Assassins- They would make great stealth ninja like creatures that can be hard to detect, can spring from rocks/water/leaf piles/high grass or even fall from tree's. Their forward movement would be pretty fast, with lethal reach, but strafing and backpedaling would be slower, making combat tricky, but not impossible.
Stinging Insects - from armored beetles with sharp frontal horns to horseflies and Hornets, there are a plethora of bugs that could realistically be a threat, offering a full range of combat techniqus to learn and also materials for early piecemeal armor types. Plus, with your skill at rendering, it'd be AWESOME to build a partial plate mail set out of the great iridescent carapaces of some beetles, or the yellow and black stripes of a yellow jacket, or fuzzy bumblebee fibers.

How about more? What else could be right fun to implement?