Designing a case to work with the range of components that can work with the ATX/ITX standards is always a challenge in the homebuilt market. The reason some of the more popular small form factor cases aren’t quite as small, is that the uber smalll attempts usually are nightmares to assemble. They require small power supply’s with specifically designed cable looms, video cards that are extra low profile, CPU coolers that are ultra small, and even then putting everything together and creating a package that isn’t a nightmare for thermal performance is challenging. That’s why dorks like me buy the biggest case they can find so that it’s easy to assemble (without slicing your knuckles apart), easy to service (something WILL eventually break), and the room for airflow allows you to run large, low RPM fans which minimize the high pitch whine that you get from small, high RPM fans.
It’s certainly tricky because in order to optimize, you need to decide what tradeoffs will be made. For example - if you limit the power supply size you can save a lot of space. Or if you require use of a special CPU cooler or small video card.
The dedicated PC builders have a better job attacking this because they can source proprietary PSU’s, motherboards, and better integrate things. The “Steam Machine” phenomenon is a good one, by being able to leverage an external power supply and mobile GPU, they were able to combine some of the desktop features and mobile features into one very small design. Not that I would buy one, but it does an interesting job of being a nicely designed ultra small form factor.
Interesting to see how you proceed. I’ve been building PC’s for over 20 years so if you have any technical questions I’m happy to answer.