I think the big advancements will be in materials and sights. Look at just the last 20 years how we have seen the explosion in use of polymers, titanium, scandium alloys. In bullets we have seen new stuff like highly advanced JHP designs, frangibles, lead free, etc.
The trend seems to be for ever more convenient size and weight balanced with recoil (light weight Glocks, Kahr and Kel-tecs), or for ever more powerful capabilities (.500 S&W, .480 Ruger, popularized .454 Casull).
So, I think we will see slimmer slides of new materials. Perhaps tungsten alloy that would have more strength so it could be slimmer, yet also stonger and still have the weight for proper cycling? Maybe Ruger will eventually make a titanium Sp101 and GP100?
Sights are a wide open field. There is already an amazing array of iron, tritium, and electronic sights. I like the idea of a remote sighting system. Sort of the speed and convenience of point/instictive shooting with the accuracy of a red dot. I think minaturization will happen for all electronic displays such as TV, computer, etc and firearm sights will follow. The largest component of any electronic viewing system today is the display. Eliminate the traditional display and get it hooked to a near-eyball projector and the sight would be a hundred times less breakable, require much less power, and would be a fraction of the current size of red dot or ACOG sights today.