They could have xur sell an elemental core for weapons like what a lot of people are suggesting. To add to that make it for a specific weapon type( scouts, autos,etc.) Have him sell a completely random one, maybe not even every week. Once or twice a month would be good. Xur could charge 50 strange coins or even a little more. Then you would have a quest to charge a weapon with that element. Something along the lines of the exotic sword quest ( the part where you get the ability kills). In short, make it grindy as hell but still let us at least have the option of an elemental primary.