Ok, so Iksar himself said on his last Q&A recap that they know extra coins on Mercenaries are a porblem and that they are "actively looking at solutions". So, here's my idea: converting them to regular gold (you know, the one you buy packs with).
Now, here's the biggest issue with the idea: what's the right conversion so that HS's economy doesn't completely break down? Well, that's not an easy answer, considering a lot of the resources for the game mode are either through packs, tasks or bounties.
- Tasks give a total of 1640 coins (for himself + randoms) + 3 packs (when task 7 = equipment, the new mercs give let's say 100 extra coins, rounding up);
- Packs give mercs, skins and coins. Now, it's not easy to round up an exact number without opening a butload of packs (and I'm not gonna spend money just to know that number), but by looking at the image from the HS wiki and my own experience, you can conclude that no pack gives out more than 5*50 (so 250) coins, unless you have all the skins and open a "duplicate skin", which will be converted to 50/75/100 coins (depending on merc rarity);
- Bounties give an assortment of coins, depending on the dungeon, but they give out at most... honestly, I couldn't find how much they give and I'm not in the mood to do a Heroic run on Alterac Valley just to check how many coins we get (especially since I don't have that many maxed mercs). So, for the sake of rounding up, I'll say they also give out up to 250 coins.
So, how much gold should we get for the extra coins? I'd say, not that much, just some extra change so your time isn't completely wasted while not being so much you'd run a bot on early game bounties for insane amounts of gold. Which brings me to my proposition: For every 200 extra coins you have of a certain merc, you can redeem X gold, depending on rarity. More specifically, 5G for rare mercs, 10G for epic ones and 20G for legendary mercs.
This way, even if you were to get 250 coins of a certain legendary merc on a Merc Pack, that would be worth the equivalent of 25G: 25% the base cost of a Merc pack, i. e., the same disenchanting to enchanting rate dust has. And also, this change helps with another small inconvenience: ever since the change to the reward track, we've always received gold in multiples of 50, which means you'll always be left with up to 45 "useless" gold leftovers if you don't play arena or duels.