Add the exchange center as a building(2x2/2x3) you can win for joining or creating a fellowship.
10 to 15 stages with each upgrade requiring decent amount of goods, coins and supplies.
Each upgrade should add two or three extra slots for placing offers
blueprints are only available from the 10th chest of tournaments. Most of the players are unable to reach that chest so adding blueprints into spire or to a chest before the 10th chest in tournaments should be a great thing.
add a way to exchange anything(except diamonds) for anything(goods, items in the inventory(except blueprints), coins, goods, spell fragments, etc.) inside the fellowship. the offers should be similar to the trader with a rating according to each items value in diamonds.
For example...