My guess is that the long-time trend towards having more, smaller photo-sites on sensors of a given size will continue as long as the floor noise levels per photo-site continue to reduce, so that per-pixels SNR will never get beyond about 16,000:1 (2^14), meaning that 14 bits will always be enough. In fact, with full well capacities apparently steady at about 1600 electrons per square micron (according to Roger Clark at
http://www.clarkvision.com/articles/does.pixel.size.matter/#Unity_Gain) once pixel sizes get down to about 3.3 microns (bigger than the Sony RX100's 2.4 microns; just a bit smaller than the 16MP 4/3" sensors), the full well capacity will be about 16,000 e- or less, so an ideal 14 bit ADC could count the electrons exactly.
This seems an easier path than having fewer, bigger photo-sites that need 16-bit ADCs, because the electron counting is done with more parallelism: with the same exposure over the same total sensor area, more smaller photo-sites leads to counting the same total number of electrons but in more, smaller bundles, using more column parallel ADCs so that 14-bit rather than 16-bits is enough, which allows faster ADC operation.