Some more info after some discussion.
- Yes, we're interested in doing a special issue coin, solid gold is on the table but we're playing with design options
- We're considering a run of around 25 coins - fewer and the premiums would be killer, more would be difficult due to the cost of the batch run and we'd probably have to get pre-order guarantees if we were to go above 25
- We can't use existing dies from previous runs because gold is more dense than silver, and the coin dimensions would be smaller, so we have to either re-make dies with a previous design, come up with new artwork, or mix in another metal to alloy with to achieve the same dimensions to be able to re-use previous dies. I mention this because it will obviously affect the pricing based on how much work/money needs to be put into the solution
- The coin would be most likely 1oz. As mentioned by others, with gold anything smaller will not fit everything needed. It won't be larger, as even at 1oz it's a smaller market due to the cost
- We have not discussed the holos, numbering, engraving/stamping or anything like that yet. I think there will be some sort of marking to make it a special issue series, just not sure what yet.
Pricing is very hard to speculate right now until we figure out more of the technical parts. If we have enough interest that we know we'd sell out, premiums could be adjusted accordingly, but we're so far away from that right now it's not worth any larger discussion.
This will not take away from our desire to issue a "normal" Series 4 coin in the traditional silver & silver/gold gilt like the past. Stay tuned for more information on that.
Please keep the ideas and questions coming!
Have you considered using gold other than 99.9 purity? There's upsides and downsides to using less pure gold, the upside being it's easier to drum up interest due to the cheaper price (and it helps you as you need less capital to produce your initial run), the downside being that people might not be as allured by the coins if they aren't completely pure.
. That would look nice on the D1. Im excited to see what comes out. These were always some of my favorite diy coins. 
