Yea, I get what you're saying and I agree that there is no real backplane or bus to entirely support an external expansion chassis directly. The integration point that I had in mind was more primative, plain old MIDI.
As for me, I'm interested in expanding its sound capability and like the flexibility that semi-modular offers, intertwining and overlayering MB sound with eurorack modules, VCV Rack, native synth engines and other sound generators controlled by sequencers, keyboards and knob noodling. Sure there's a bunch of eurorack modules on the market which offer the capabilities that I'm interested in, like 0-coast, lifeforms sv1, Neutron, Mother-32 and the list goes on, plus the various Arturia mini & micro line up. The Rev 2 has potential but it doesn't fall Into the semi-modular category but it's a step closer to a performer device vs a plain old Integra-7.
Guess as subtle as it was, was saying to Arturia that they have some customers who want to expand their MB kit and is willing to share their basic requirements and money. Will I see a product anytime soon? I doubt it, and before I buy more product I do want the MB to be in a fully functional state.