Well, when I've first thought about a sandbox campaign which would later become Barbaric Frontier, I wanted a game combining the feel of first edition WFRP, all it's intertextuality, grittyness, dark humor and moorcockian weirdness. All this pitched to its extremity. All because my big disappointment caused by the following editions of WFRP. Also being a huge Conan fan I couldn't resist to place some bits from Howard's Hyborian Age, that is the aquilonian/cimmerian border with pictland, hence the Barbaric Frontier and Skraelings. This was a straight way to incorporate some cthulhu mythos. This part of inspiration was to satisfy me as a GM. However my players do not share my literary/rpg tastes, so I had to familiarize the whole thing up. Hence the vanilla fantasy bits like fantasy races(but with their own twists), barbarian/norse-like people, and some typical dndish menagerie.
When it comes up to the visual representation of Barbaric Frontier, I see it through the lenses of Frank Franzetta's Conan and other sword & sorcery art, brilliant Fighting Fantasy art of Russ Nicholson, and polish illustrator Jarosław Musiał(check my inspiration posts for his brilliant line art, very old-schoolish).
The main theme of this campaign is struggle between civilisation and barbarism. Decadence of civilised institutions, superstitions of common folk, evil cults' plots are nontheless strongly emphesized. etc
PCs in this camapign should be scoundrels of some sort, people from the outside of conventional social structures. Peasants, who fled from their masters' manor, thieves, mercenaries, non-firstborn sons of knights and nobility, barbarians, disgraced clergyman, wandering monks, self-righteous witchhunters, whom have questioned the authority of the Chuch of the One, all kinds of sorcerers, alchemists, thaumaturgists and other heretics.
So to categorize the Barbaric Frontier I'd say that's a cosmic-horror-sword-&-sorcery-dark-humor-dung-age-weird-european-fantasy game :p. Sorry for my poor english and for the length of this post.