Electric will surge in popularity. Nitro will not go away but even the most diehard nitro burners get frustrated if something happens to their $300 engine that they could not have prevented and no matter what, it happens to everyone once in a while. Nitro's are also finicky to tune and noisy which limits the venues where they can be run.
I think the scale looks of the Slash class have a future but I don't think buggies and truggies will go away for the same reason that guys can't leave their Slashs stock. Performance will almost always dictate the evolution of our toys (with touring cars being the glaring exception). I'd like to see Truggy class go away and become more of a scale class like the new Slayer, except with buggy/truggy architecture and more of a spec style, ie. slower engine, less grip/spec tire.
A big limiter of offroad right now is property access. 1/8 nitros are noisy and require a huge chunk of land to race well. A slash/slayer type of class would boost realism and with a slower platform and tires that wouldn't disentigrate on pavement we could get back to "dirt lot" type racing where part of the track could be in dirt and part could be on pavement (I'm thinking of those peices of land that exist on the outskirts of typical mall properties). The hobby would be closer to crowds of people that weren't seeking it and you'd then have 100's of kids begging their parents for RC stuff, like they used to.