My thought was more along the lines of the ThinkPad UltraBase (ThinkPad X4 UltraBase - ThinkWiki), but using an expansion port rather than the proprietary dock connector the ThinkPads use. This way you can have a DVD drive with the machine as well, without the clumsiness of having some external drive dangling around and in the way. You’d have an extra expansion bay in the “FrameBase” (to coin a term for it) to keep the module you pull out to attach the base.
Personally, “thin and featherweight” is FAR less important to me than a machine that has all the necessary capabilities. My current personal laptop still has an optical drive, and I consider it a necessity for my use cases. Especially if you’re dealing with large-scale systems that provide their documentation on CD/DVD.
Biggest issue would be how you “clamp” it to the Framework laptop, as you’d want it to stay solidly attached while carrying it around.