JMHO, but I think the speeds are scraping the ceiling already. Lighter arrows and bows tough enough to shoot them long term would be the main way to add speed. The fastest bows out there are already about as efficient as they can be and have very tough draw curves. Where can you add more energy? Unless a bow has some kind of ratcheting mechanism, where would the speed come from? It would be nice to see manufacturers concentrate on making fast bows very solid and reliable, but smooth and pleasant to shoot. Really, to survive in the market, a bow maker must find something each year to put out there, and make customers feel like they can't live without it.
That is the future as I see it.