I wounder what the mechanical interface would be.
Given the approach to "low cost off the shelf components", I'd assume one might use ISA connectors for that, since they are still being made and widely available at reasonable prices.
In order to prevent someone from sticking in an ISA card, which will be incompactible, it might be useful to insert the slot 180° horizontally rotated and have it aligned so that expansion cards are oriented like PCI/PCIe cards, making installation of ISA cards impossible.
This way, production of said expansion cards would also be cheaper - espechally when they do want to provide I/O outside of the case [like a network controller/"transparent modem"]...
Also since ISA edge connectors are relatively big and chunky, they also lend themselves as a good basis for people who might want to DIY and prototype said expansion cards.
Not to mention that additional pins of a 98-pin connector might also allow additional voltages or connection options to internals - i.e. IEC or joystick ports or serial ports or even audio, thus making modifications even easier as well as allowing more modularity and upgradeability without sacrificing the minimum viable goal.
Personally, I do know someone who might be interested in developing some FPGA-based accelerator for the X16 and having some expansion interface ready early on will greatly support both hardware and software development as well as making an independent aftermarket possible.