Me thinks several trends might be worth to watch in this regard.
1. Technologies are achieving closer towards the resolution limit of human eyes
For example, retina display which is at 326 pixels per inch, fairly close to human eye's limit except the fovea.
Let's pick 36 MP image size, it is about 7350 X 4900, in 24"X16" prints it is 306DPI (printers actually need much higher DPI to achieve this due to dithering).
If put 36 MP into full format sensors, it will provide resolving ability in the area near 200 pixels per mm, or 100 line pair per mm. (current crop models have higher marks here if the SNR is not the first consideration).
Current top glasses available in the market can provide resolution around 65 lp/mm. The optics is not really in the close range of 100 lp/mm, but it's in the perspective already. (I guess the size of lens is more related to the exposure quality rather than the resolution quality. Manufacture precisions like the surface accuracy of aspheric elements and the assembling precision of elements inside the lens probably will be more important in this regards).
My take of this trend: we probably will reach a point very soon that we may have enough megapixels for most of regular use cases, thus a crop model might have some advantages other than image quality over full format.
2. Sensor technology are proving the IQ advantage of larger sensor format will become less significant and evolve towards marginal.
D800 has proved a 16MP 1.5 crop model can have similar IQ as FF. Here 16MP is the 1.5 crop of 36MP.
16MP is what 1D4 can offer, very enticing. Will this number climb up further? Very possible, though the simiconductor technology is rapidly approaching the limit of the materials. I have a feeling we probably have much more room to improve in the DR direction than the pixel density. I guess a 24MP or even 30MP sensor with similar FF image quality in five years might not be illusional.
My take of this trend: more quality crop models obviously. However, once crop models have attained quality, it have a big impact in lens design, which is, few long range heavy big whites, more challenging wide angles (white as well? ;-)
3. Mirrorless technologies are still in their early stage. Currently it is still very difficult to call any technologies available now a real competent OVF alternative. However, what the removal of the mirrow could offer is attractive, a lighter, more compact quality body for the mass population.
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In my vision, a moderate crop factor pobably APS-C, with reasonable decent image quality will become prevail in the next few years. If mirrorless front could bring up any breakthrough, then mirrorless will join the evolution and call for a new lens mount. However, it will take at least 10 years to build up a full spectrum of quality lenses for a new mount. Therefore, full format and SLR will survive and thrive for quite a long time before it is squeezed into the same camp as the medium format (if MF is not dead beforehand due to lack of R&D budgets).
I know lots of members in this forum might prefer sizeable bodies which can be better balanced with their top glasses. However, what I am pointing out is the trend in the mass consumption (though it's fairly small and targeted) which is usually the decisive factor of product evolution. 40 years ago, few photographers will place serious weight in 135, but it was the winner after all.
