Supposedly Audi has greatly improved the efficiency of the automobile CVT by putting some kind of metal chain inside the rubber transmission belt. I have no idea whether a smaller, lighter, version of that will ever be viable for bicycles but it sounds like it might.
Personally, I think that conventional derailleur systems could be improved a lot. One of the ways a person might do this is to eliminate some of the duplication of gear ratios (large-chainring, large-cog giving same ratio as small-chainring+small-cog). Another improvement for those of us that commute in bad weather would be to cover the gears up so they don't get snow/rain/road grime on them.