I'm sure you have already considered this, but I want to highlight market segmentation. I think niches are important to find and inhabit. What that means here is, try to be at a good size for your feature set that no other case has. Have the most compatibility per litre. The larger your case, the more competitors you have, and you'll be more directly compared by other factors like price, availability, shipping time, quality, etc. I loved the A4 because it fulfilled its niche well. For my next case, I am looking at 3-slots, air only (so classic layout for CPU cooler), from a reputable manufacturer, and as small as possible. At the moment, C4 looks very attractive, with the added bonus of the innovative upside down orientation. But, the bigger it gets, the more other products I have to start considering.
I do agree that a limited availability of the compatible cards does hurt the usability of the product, but at least they exist. For this generation at least, I think the combination of crypto's newfound unpopularity and the price point of the 4000 series should lead to a better supply situation.
I hope this feedback helps, and I am excited to see what you create.