I think bighorns would be a great species to make into a Great One.
With them being split into two subspecies I think due to how little physical changes they have that they should "share" the great one variants. The Rocky species would have more visible fluff around the neck and back while the deserts would remain un-floofified.
Sheep/ Goats have lots of variability with their horns which leaves room for tons of different horn types to hunt for. Both species would share the horn/ fur types but I think having each species have 1 unique fur/horn would be nice too. Below is some examples of horns I would like to see.
With the addition of the sheep G1 should also come new multimounts.
Hot'n'Cold [1 Male Rocky, 1 Male Desert.]
Headache [1 Male Rocky, 1 Male Wolf. ] (Wolf getting clowned on by the sheep in majority of the poses)