New building or shop option - Alchemist
On times there is always a desire to have a little more of one resource just to get that next upgrade.
The alchemist option will allow 1 resource to be exchanged for another and the ratio will improve as the player improves
So a beginner may start with a ratio of 20:1 for gold and elixa and a maxed payer may get 2:1.
The ratio improvement can be tied to:
Town hall level
Wizard level
Players level
an alchemist building which will level up by paying with gems
Or a simply a shop option with a sliding ratio level bought with gems.
If an alchemist building is added to the game there is no reason for the conversion to be immediate. Also all resources being converted could be plundered and lost if attacked by another player so making it a building worthy of defending.