Here's my attempt :
Sky&sun system
Current sky systems in unbiased renderers are so limited. Yes they are likely to be physically correct but they give you only 5% of what skies can look like depending on different atmospheric conditions. If we look at all those terrain apps like Vue, Terragen 2 we can see that their sky systems are soooo tweakable. I'm not sure about the physical corectness of those sky systems but with some tweaking you can create hunderds of different sky gradients + sunlights which can always look super nice and super real. Much more real than the current sky systems based on that single paper.
Displacement
A feature that slowly crawls to unbiased renderers. Displacement is definitely future because with the processor developement we can start switching from bump mapping to displacement. Bump-maps only work for low surface deformation and it heavily suffers from the fact it cannot shadow itself. The new maxwell's non-polygonal displacement looks like the best implementation of displacement in a renderer to date. This approach sure smells like displacement's future. Also vector displacement support is the future for all of us high-tech displacers.
Material System
While the current material models are cool they're kinda not that flexible. I would really love to have the option again to set the reflection through a simple vertex controlable falloff curve. The basic idea is the ability to go way beyond physicaly correct properties of the material. Who cares if it's not physically correct when it looks good
+ It's great to finally see the phong energy loss is being solved.
+ The abbility to read volume 3d grid datasets (let there be smoke, or clouds)
Tone Mapping
A renderer spits out an image which is raw as it gets and then you have an editor with like thousands of different sliders, tweakable curves, histograms for everything possible so you can tone map your ass off it and create all kinds of different looks. Again the ability to push it way beyong the borders of looking real.
Aeronoise™ (implemented in Fryrender by Feversoft)
A mandatory feature! For those who don't know it, it's a feature that lets you paintbrush a selection area over the rendering to tell the CPU to only focus on those parts of the images. It's an insane timesaver for areas that clear slower than the rest. Maybe some kind of a percentage feature would be cool too. I see some area of developement in this feature.
Render channels
Not much to write here. From alpha channel to everything else.
Real time preview
Radium renderer had it and now Vray is going to have it. Simply a perfecto feature. Imagine changing tweaking the params and having an almost direct feedback on it.
Speed
Well I don't care about speed if I can get what I want. Btw still hadn't heard a single statement about the Coherent MLT with Multiple-Try Mutations ( http://bat710.univ-lyon1.fr/~bsegovia/p ... f/cmlt.pdf )paper from a renderer developer. Is that technique worth a try?? Ian, Radiance, Chema, Ono .. hello??
Fin
I really should switch to linux 
