Computer science and information theory were separate from physics. Not anymore. People realized that information had to be physical and this had profound consequences, especially in the form of quantum information/computation.
Psychology and economics were separate. Not anymore. People realized that humans were the core of economic systems and their behaviors fundamentally shape the nature of economies, even at the largest scales. Note the rise of behavioral economics.
Neuroscience and computer science were separate. Not anymore. People realized that thinking about the brain as a computer is probably the best possible abstraction to understand it.
Reality exists. There are no intrinsic boundaries in reality. All fields of study are created by humans. But these divisions seem so natural that nobody realizes that the boundaries have to dissolve. The fields have to collide. And when we realize that--or finally have the language and ideas to meaningfully talk about it--we find out all of kinds of crazy, cool stuff.
So: what collisions are we currently blind to?
A potential boundary is that between the seemingly objective and subjective. At the moment, science seems to be set on measuring things outside of the self. There is relatively little exploration of what one subjectively perceives. This facet of existence is almost completely ignored, even in psychology. I think in time this boundary will have to dissolve. There really is no good way of separating the outside world from the inside world.
Another one could be the artificial boundary between nature and nurture. It seems like the expression of all genes is mediated to some extend by the environment, therefore it is senseless to talk about any boundary between the two. I recently heard a metaphor that the debate between nature and nurture is akin to debating whether length of width is a better determinant of the area of a rectangle.