Well I think the future will bring new technologies in two distince but different areas. In recent years there has been a great advance on launch coaster technology. Research will be spent trying to make these as effiecent as possible. Its been shown that any given launch device can send a train at high speeds in short time, each device has its own setbacks, be it power (lim) or a cleanliness (hydrualic) factor. I see it very possible that these launch features become modified to replace chain lifts. This is already true for California Screamin and Millenium Force, which both use technologies that can be found within a launch coaster.
Then theres the other camp of where technology will push the limits....how many different ways can be found to ride a coaster. The world went from only being able to sit on a coaster, to standing on it, to inverting the track. The 4D offers the most versatility when it comes to riding, yet only one has been built (with a rumored 2nd). The current 4D design definatly needs some overhualing, which is probably why one hasn't been built for a while. I think its fully possible that a 4D will be developed that is a much much more scaled down version of the Arrow version, those trains are just massive.
As for all other riding styles other than 4D. Its not entirly out of the picture of parks to retrofit current rides with new trains. I know its been widly discussed that people want to change the Flashback trains at SFMM. And I know that CP toyed with the idea of putting floorless trains on the Mantis, but it was deemed to expensive to retrofit. But a park that really wants a new ride experiance out of an old ride, this isn't entirly out of the question. Innovations in the design of the ride should start to hit on, new inverions probably will be invented.
THen theres wooden coasters, which I doubt will change that much becuase, well, people like them becuase they are nostalgic. Although more extreme elements will start to become a trend (see Hades).