Well, there are big areas where the ecosystem is quite underdeveloped, like differential equations, which have a lot of holes that need new algorithms and improvement. It would be extremely difficult to develop all of the necessary algorithms in C/C++/Fortran, and pretty much impossible in Python/MATLAB (I tried at first), but it's a breeze to tackle this in Julia. So for these kinds of scientific computing areas where there's tons of work with few people with the necessary expertise, Julia is a great way to start getting some good implementations out there for people to use.