Was it an idea to have an option to convert coins you have in overflow, to a coin to a merc you allready have. Not as it is used to make a new merc, but to boost those who need coins.
There is no use for the coins you have to many off. So maybe in your hub, you can convert them to one of your other mercs. I have 3000 Cariel coins I could use elsewhere.
Just an idea, to implement more stuff to the mercenary mode.
They did say that they were gona add a use for the extra coins but have been silent ever since 
They should allow to:
- craft portraits using dust
- convert unused coins to dust that you could use to craft coins
- ideally, enable smart loot so that you are guaranteed to loot a subset of coins for not maxed heroes in your packs and when ending a bounty.
Regarding the crafting I would say
- Sell 4 rare coins for 1 dust; sell 4 epic coins for 5 dust; sell 4 legendary coins for 20 dust;
- Buy 1 rare coin for 1 dust; buy 1 epic coin for 20 dust; buy 1 legendary coin for 80 dust.
By doing so, when you sell something, you are payed back for 25% of its value.