I know a desktop or rack version has been discussed in a previous thread, and I think the conclusion reached was that Montage is to much of a perfomance instrument for that to happen.
As a synthesist, i.e. not a traditional musician, I personally would want a desktop version with the full synthesis engines, FX, and all of the performance controls. But scrap the arpeggiator, performance recorder and all the rest of it, and I'm not sure I'd need all of that sample memory either ...
The transport controls for the recorder/player feature would go away - this is normal for the rack versions of the past. All of the bank buttons too. Mast rack units haven't kept all the performance controls, though. The XS rack didn't have any of the assignable function switches used for XA. Given the touch interface, it'd make sense to make a multi U (multiple rack unit - not a "skinny" rack like the past) rack enclosure to accommodate a touchscreen. Something like the EX5R without the floppy, smaller area for the assignable knobs, touchscreen, and no transport controls. EX5R only had two scene buttons (SC1, SC2). I would consider making the continuous knobs of the assignable knobs also have a tactile switch when pushed. Therefore, these buttons could double as the scene selector when pushed saving room by not having to have another area for scenes.
You also do not get sliders so "full performance control" should be thought of more in the context of what MIDI controllers generally have. And knobs or sliders for are on most MIDI controllers - so probably no need for that performance controller to be on the rack. Same for pitch bend and modulation wheel.
Superknob would probably want to live on the rack too since it has SysEx mode. Probably shrink this down to the same size as assignable knobs is fine. Tactile switch on the knob would I think invoke the "assign" feature of assignable knobs - and maybe other buttons switch these out for the EQ/FX (etc) functions.
I'm not sure why the audition button, as on the XS rack, would need to be there. But past gens have shown Yamaha likes to include this.
With the touch screen, lots of choices are available for throwing overboard buttons/knobs and favoring forcing the touchscreen.
Although the options currently available - between an external touch screen and a MIDI controller - you could tuck away a Montage 6 somewhere behind the desk and use a smaller footprint keyboard (or none, just use the computer). In which case a tiny MIDI controller (no keybed) with some knobs could be used for some real-time superknob/assignable knob/etc turning. As your use case has a computer in the middle I imagine so the knob mapping using software is available.