Oh, I love this question.
Clearly, bit-depth and sample rates are up for an overhaul.
However, when you folks consider the bit-depth portion, I hope you will also take into account DSP issues, communication issues (latency), truncation elimination (if possible) and total computer thruput...
For example, say you chose 32- or 64-bit. Some could argue that some of the extra bits were not really necessary...but others might also make a counter-argument that the elimination of any and all intermediate translation steps would have offsetting benefits, and therefore, 32- or 64-bit may make a lot of sense in the long run.
I suppose another way to view this would be to say something like "Future formats need to take into account overall system perfomance issues."
My thanks to the orginal poster of this question for asking it.
Taking the time to get the future formats "really right" should make all the difference in the world.