A new guild won't hurt.... 

Anyway....

DS, Dragon Slayers 
GS, God Slayers
DeS, Devil Slayers

Not that it is banned but it needs restrictions. DeviL Slayers is less op hax cuz if I remember, Silver exorcists Demons by eating their souls wait I take that back, Devil Slaying Magic is has cuz not even Natsu broke out of Silver's ice.