I really like this game and it is really promising so far! I'm not exactly sure on what you have coming up the pipeline idea wise but here are some suggestions from me.
1) - Make the player have to purchase mines
In the beginning of a game, all mines would be locked. This means that the player must purchase ingots and have them delivered, but when the player makes enough money they can purchase a mine. Have the mines scale with price. For example, the cheapest mine would be a copper mine that the player would be able to purchase first, then the next would be a tin and iron mine and so on and so forth. This would add another goal for the player to work towards.
As a side note, later you can implement workers that the player can hire to work in these mines that would deposit any ore they mined outside the mine for the player to come by and pick up occasionally.
2) - Have the ability to hire and outfit guards / have theft events
Now before I get into this, no I do not want any combat mechanics in this what-so-ever, I believe that would detract from the spirit of the game. I do however believe a theft/burglary event of some sort can add another fun element and obstacle that the player can overcome. Perhaps every night or when the player goes to sleep, there is a random chance that a burglary can happen. When it does happen, the player will awake to a message on their screen saying something along the lines of that there was a break in and lists the items/money that was stolen. The items that are stolen can be random somewhat, but giving weight to the type of item and the cost of it. So for example, if you have a lot of copper ingots but you also have a handful of iron ingots as well, the thief will go after a couple iron ingots before the copper. You can work out the mechanics exactly but I just wanted to put you in the same place I was thinking.
Now to minimize the risk of future burglaries the player can hire guards from the castle. The guards would have different stats that the character can use to determine which guard would be better. Now, the more better the stats, the more expensive the guard is to hire. After a guard is chosen, the player must equip them with armor and weapons. The better quality the weapons and armor, the better the guard is....or something along those lines. You could also hire more guards to guard any mines you've purchased too, because those can be potential targets as well!
3) - Have better ingot organization
This is more of a quality of life thing, but trying to organize and put away ingots just seems a bit too tedious and awkward when trying to put them on a shelf or something. Perhaps have the ingots snap to each other so that they will automatically orient themselves next to another like ingot, or have special shelves that the player can make that has a grid that the ingots can snap to, much like they do already with the anvil. It just takes a bit too much time in my opinion trying to orient ingots so that they all fit nicely on the shelf as it is now.
4) - Be able to build a cart for your horse / yourself
With the ability to craft shields and what not, it got me thinking why can't you also build a cart for your horse? There could be different carts that you can build from a small cart that can hold a max of two boxes and can be pulled by the player much like a rickshaw. The medium cart can be like the cart that is in the game now, but can hold 4 boxes and requires a horse to pull. The largest cart should be able to carry 6 boxes and actually require a 2nd horse. These carts would obviously increase with price as you upgrade and would require various metal and wood parts to build.
Well I know it is kind of a lot to read, but these are just some of the ideas that I have thought of while playing your game. This game has a lot of potential and I know you probably have a lot of ideas on where to go, but I hope that you possibly consider one of my ideas. At any rate, this is a great game and I know that whatever direction it goes, it will surely be fun!