One thing we haven't seen in graphics pipelines yet is good (any?) support for open-ended lighting (we have floating point RGB values, but they don't deal with brightness beyond 1.0). Maybe this is because John Carmarck only does dimly lit game engines... An example of this is when you emerge from a torchlit cave into bright sunlight, the outdoor scene ought to be blindingly bright until your eyes adjust. When you look into the torchlit cave from outside it should look utterly dark.
We've experimented with that at Epic, but it just isn't fun. I think we've implemented it twice now in various engines for various game ideas and it just doesn't feel good. It always comes off as an annoying hinderence rather than any kind of "oh neat!" optical effect.