Those already exist but it's kind of missing the point. An NVMe SSD is the size of a stick of gum, I'd much rather plug one of those in than have to stack a bo

Those already exist but it's kind of missing the point. An NVMe SSD is the size of a stick of gum, I'd much rather plug one of those in than have to stack a box the size of the Mini on top of my Mini and connect it with TB3. In the end you've got something more expensive, probably slower, and less portable than the obvious solution.