Ok, I have been nerd dreaming about your design and here are a few things I came up with by trying to be as close to possible reality as possible:
So I don't think going modular anything is a good idea, because if you don't know what elements a controller is gonna have it will be difficult to make a general script that uses them.
I also think having it possibly control two instances of Live is out. I don't think there could be a software script that works in a way where both instances of Live have full control over all the elements at the same time and if you make it a split aproach it will make it not flexible and you would be better off with two Pushs.
End of the buzzkill
Now to what I think would be doable (even if a lot of programing on Abletons part would still be needed to make it so).
(A few minor hardware additions would be needed to your design, but not many.)
The 16 display encoders:
Since Live generaly has 8 macros on its racks and 8 parameters per bank this lends itself to three different modes :
2 device, 2 bank and 2 clip control (I think there should be extra small buttons to switch between them)
In 2 device mode the 16 encoders always map to the 8 macros each of two neigbouring devices on a track and in 2 bank mode to two neigbouring banks of one device.
(And in 2 clip mode to two clips on neigbouring tracks in the same scene)
Since two independent blue hands for navigating are probably hard to program, the next best thing is to just be able to lock the first 8 encoders to a device/bank/clip independently from the other 8, so that you can first lock and then continue navigating to lock the others. I think there should be 2 small dedicated lock buttons to do that.
The 16X8 grid: (Many things)
1: Split grid mode: (To control 2 different devices or clips on different tracks with the 16 encoders will be very usefull here)
1a: 2 times 8x8 to step sequence two different clips at the same time. (for this again there should be two small permanent buttons to lock each segment to a clip or resolution mentioned in 1b.)
1b: also a split grid but with two different resolutions to use on the same clip
I would suggest a second row of the resolution buttons (they could make the resolution buttons smaller than they are now though) to make the two resolutions selectable independently (these will perform different functions mentioned later as well)
2: The same as mentioned under 1a and 1b but with the grid split into two 16x4 grids
3: the obvious 16x8 control of one clip (this is finally the perfect grid size for a step sequencer IMO)
3a.) this is a size where arrangement step-sequencing becomes useful. For this though, larger grid sizes to select than 1/4th become necessary. These should be functions of the second row of resolution buttons as well. (via shift for instance)
In arrangement it will be much more important to move the focus of the step-sequencer along the timeline, for this I would suggest to use the flat buttons you have placed directly below the grid. They should display the playhead but also allow for shifting the step sequencer focus by pressing.
4: for keys: 16 keys cover 2 octaves in scale mode of pretty much all scales which is often plenty for specific parts.
So I think each row of 16 should be independently assignable to a track, note range and scale. Imagine having 8 different instrument parts in the correct scale and range to play on the same controller at the SAME time! (Or 4 covering 4 octaves each etc.)
For the two rows of resolution buttons there comes another nifty function now: Two independent NoteRepeats either on the same instruments (different pads of DrumRacks are the most obvious use case here) or on two instruments. What would be important for that would be another independent swing knob and an independent NoteRepeat button.
Love the joystick: I think to do it justice every rack and Live instrument should get the x-y grid that VSTs get with the joystick always automatically mapping to the x-y pad of the device in focus. Since controlling two different devices is now possible (as described above) I would add another joystick replacing the 4 encoders you placed on the right. Also, just as on the left, add another crossfader above the new joystick.
One hardware addition I would also make (since you have been so liberal with LEDs already) each of your faders should have 12-16 level indicator LEDs next to each fader.
Make the lower body plastic (like Maschine) so that it's not too heavy and give it a sturdy cover/lid so that it can still be carried safely to gigs despite the size.
I know this will stay a nerd dream, but I would save up and buy this for up to a 1000 bucks, if it comes bundled with Standard or not.
And dear Ableton, if you build this, I forfeit any right to the IP in this post
Shut up and take my money!!! Nerdgasm..... 
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