Concept art (by Frontier's Ben Andrews) of another SRV variant (but I guess this was a design that evolved into the one we already have):
http://www.edassets.org/img/concept-art/horizons/srv/benandrews_srv.jpg
Edit: Source is http://benandrewsart.com/project/
That looks like a more proper cargo-focused SRV.
A mining SRV would be cool, with a turreted mining laser and some more cargo capacity and fuel capacity in exchange for less agility/speed than the Scarab. Call it the Badger or something