Coeffects: The next big programming challenge
Many advances in programming language design are driven by some practical motivations. Sometimes, the practical motivations are easy to see - for example, when they come from some external change such as the rise of multi-core processors. Sometimes, discovering the practical motivations is a tricky task - perhaps because everyone is used to a certain way of doing things that we do not even see how poor our current solution is.
The following two examples are related to the work done in F# (because this is what I'm the most familiar with), but you can surely find similar examples in other languages:
Multi-core is an easy to see challenge caused by an external development. It led to the popularity of immutable data structures (and functional programming, in general) and it was also partly motivation for asynchronous workflows.
Data access is a more subtle challenge. Technologies like LINQ make it significantly easier, but it was not easy to see that inline SQL was a poor solution. This is even more the case for F# type providers. You will not realize how poor the established data access story is until you see something like the WorldBank and R provider or CSV type provider.
I believe that the next important practical challenge for programming language designers is of the kind that is not easy to see - because we are so used to doing things in certain ways that we cannot see how poor they are. The problem is designing languages that are better at working with (and understanding) the context in which programs are executed.