I would think that whatever consumer processors Dolby and DTS come up with will only scale to around 20 some speakers and they'll probably use the 7.1 + two overheads + LFE configuration and then it will be up to the consumer to fill in the gaps if he/she wishes for a much more enveloping experience.
The best thing to do would be to have a base Atmos/MDA unit that had at least 9.2 (7.1 + dual overheads + dual subwoofer outs) or 11.2 outputs (base 9.1 channel bed config. + wide L/R side surround output) and then you could attach a module that would link to the unit digitally for command and control, allowing you to add the rest of the speaker outputs for the full effect.
Though, the soundtrack would still contain information for even more rendered speaker locations if they decided to come out with a processor capable of the complete theatrical experience for more sophisticated and larger home theater viewing rooms.
The scalability would still be there no matter what and the studios wouldn't have to keep making more versions just to add more "channels" as they do now.
That's pretty much how I see it too. The greatest selling point to the consumer public IS that it is much, much more flexible wrt speaker placement compared to any other discrete channel digital audio delivery system. However, there is still a standard for a surround layout and that's what will be used in the mixing studios and that's what we will continue to replicate at home.
Blah blah blah. I want my overhead surrounds!!!!