Considering the limited amount of energy I think it's not worth making a whole new activity just for gold, especially since Hunts give a good amount already on top of being the primary activity for getting gear.

They can simply remove the unequip costs nonsense completely, because it's just that, pure nonsense, and stop forcing us to waste resources just to see if a piece of gear is good or not. Artery Gear solves this issue by fully rolling every piece of gear when you get it, but to unlock the substats you need to upgrade it. So, bad substats? Fodder gear. No need to +6 or +9 a piece to find out.

Now, a dungeon to get charms reliably, that sounds much better imho. 

Hunt give 30k gold, and you get 40k+ gold ( when 30% gold increase buff time) 

Epic equip sell for 25k gold. 

Right now that is the way to achieve gold. 

Or the special sudden quest in adventure which gives 100k gold. 

So if you ask there to be gold dungeon that provide 100k per entry run, i think it is almost impossible. 

Yeah gold are scarce right now, gold never enough if you want to upgrade everything. Using + 15 to enhance others u need 600k. So that is a lot of goldd

If you're running out of gold, you're doing something wrong. While they should definitely remove unequip cost to alleviate that gold sink, they made pretty good adjustments to the cost of upgrading last year. 

If you're running out of gold, you're either +15ing too many things at once/things you shouldn't be +15ing anyway or you're refreshing shop too frequently. I've literally only run out of gold once in the past year since the update and that's because I refreshed all of my skystones for mystics and bookmarks. I craft and upgrade constantly, too. 

we just had gold dungeon 1-2d ago

got 30m gold roughly from there, hunt buff it is

In the early days there used to be a gold dungeon and (I think it was) a charms dungeon. It was the Training Grounds Charles mentions in one of his lines. I highly doubt they'll go back to it. You'd have better luck asking them to bump the gold amounts in drops in adventure/hunts/Expo/World Boss/Tower/HoT. 

As noted, the best bet is going to be just farming hunts. Once you get to 13s, you pull down 33.9k gold a battle. Penguins still sell for 6k, 18k, and 54k a piece, respectively, so penguin sales are still a good cache, especially w/ how easy they are to come by now. Gold is not really a problem at all once you realize just how much you're wasting on stuff you don't need to be wasting it on. Once you get to late game and you aren't blindly gearing every hero you get with any remotely suitable piece you've +15d just to pack in the stats, you'll have far more than you'll reasonably need.