Pentax LX, Nikon F3 and Canon F-1 all had replaceable backs. Options typically were dial data backs or large film magazines that held 250 exposure rolls.
They also had changeable finders and screens, a marvelous era for 35mm film cameras.
Anyway, in my daydreams, a digital back for that type camera would need sensor, processor, iso control, display for review and batteries.
The camera body would retain the shutter, metering functions, aperture controls.
I cannot imagine live view or video, that may be a limitation of my imagination.
Linking iso settings between the camera body and the digital back also eludes my imagination though manually synching the two could be pretty easy.