ggplot2 is now stable, and is unlikely to change much in the future. There will be bug fixes and there may be new geoms, but there will be no large changes to how ggplot2 works. The next iteration of ggplot2 is ggvis. ggvis is significantly more ambitious because it aims to provide a grammar of interactive graphics. ggvis is still young, and lacks many of the features of ggplot2 (most notably it currently lacks faceting and has no way to make static graphics), but over the coming years the goal is to make ggvis better than ggplot2.
The syntax of ggvis is a little different to ggplot2. You won’t be able to trivially convert your ggplot2 plots to ggvis, but we think the cost is worth it: the new syntax is considerably more consistent, and will be easier for newcomers to learn. If you’ve mastered ggplot2, you’ll find your skills transfer very well to ggvis and after struggling with the syntax for a while, it will start to feel quite natural. The important skills you learn when mastering ggplot2 are not the programmatic details of describing a plot in code, but the much harder challenge of thinking about how to turn data into effective visualisations.