Supercell should create a new building.
The Market place where you can exchange ressources.
They could join this with the gems and ressources store...
Upgrading the builing lower the exchange rate and raise the exchange limit per day.
Exemple :
- 100k gold for 75k elixir vice versa
- 100k gold or elixir for 1k black vice versa
Something like the good old Age Of Empire on PC for those who played.