I would definitely not class Fyreslayers as being original, they are simply WFB Slayers ported over and expanded into AoS. That said, Slayers were wildly popular back in Fantasy so it was a great move for GW to expand them into an AoS faction. I'm sure they are quite popular now. Personally, I think they are okay. The faction has virtually no variation within it. Look at the variation found in Seraphon or even the Kharadron Overlords. So while I am not particularly a fan of the Fyreslayers, I can see why others would be drawn to them. Personally I think the Daughters of Khaine are a much cooler army.