Cannondale with it's new Boost 148mm rear spacing standard could fit some more gears theoretically without making the chain thinner.
But the real question is if more gears are actually needed. What really matters is the cassette range which is determined by the smallest and biggest cog wheels.
Now we have the smallest cog with 10 teeth, and there was a 9T-36T Microdrive cassette from Canfield Brothers, but it's discontinued because the torque transfer on such small sprocket size is very inefficient. So I doubt we will see 9T as the smallest cog.
There could be a bigger cog-wheel added though, for example 44T Sprocket which OneUp already offers as a replacement for 42T one. Such 12-speed cassette would be heavy, even 11-speed cassettes are heavy. Even with 44T as the biggest sprocket I feel like 11-speeds are enough for smooth shifting transitions. I don't think 12-speeds are needed. Maybe only for marketing purposes..So it's possible some brand is secretly working on 12-speed setup.
Edit: Yeah so it seems the 10-42 gears are not enough so here we have the SRAM 12-speed Eagle drivetrain (rumored):
And shimano is not behind, there will be a 12-speed XTR too (picture from BikeRadar article - not real):
Soon we will wrap the chain around the tire:)
What actually could be the next thing and what we know manufacturers are working on:
gearboxes: they offer more range than 3x10, the shifting is super-smooth and precise, the system is sealed against elements, it uses a silent belt and the weight is in the right place and not on the wheel. The downsides are now big weight (only metal no-one making it carbon-fibre) and big cost. Both these negatives have huge room for improvement.
hydraulic shifting: we see this already on road bikes and on some mtb. Easier and more precise shifting. Could be the same revolution as with hydraulic brakes. The cost is high because of low volume production now:
electronic shifting: we see this already on road bikes and on some mtb. Shimano Di2 is an example. Faster more precise shifting but more weight and battery dependance. We see also some electronics in suspension now, bluetooth sensors, bikes are getting more battery tech:
So this is hopefully what will come in near future to the mainstream. It will change the game and shifting so much, that I think we won't look on shifting the same. Thinking about 12-speed cassette might be the same thing as thinking about larger rim-brakes when disc brakes debuted.