Hey, I really like this idea, in concept. As long as the exchange rate was low, I think it's fabulous! It would alliviate a bit of the multiplayer mission greed. That is to say, One guy wants water spells, so he does 4 missions for sea essence. Well, that sucks for the other players, who wanted earth essence! It might require some rejiggering of the "master inventory" system, but I think it would work out well. The only downside is that since the exchange rate would be low, you could effective waste a lot of resources. For example, turn 8 umbra ember into 2 earth essence, but then you finally find that mission which rewards earth essence. Oops.
Still, I like it.