I wish people would focus more on PROGRAMMING rather than LANGUAGES. A better question might be, what area of computer science do you want to learn next?
Languages affect how you can express certain concepts. More expressive languages can be useful in new programming concepts. A personal example: Many years ago I had to program in VB.Net, and never understood polymorphism until I dabbled in Python and played around with some projects in that language. VB.Net lets you do Polymorphism, but it doesn't assist you in learning it (at least for the way my brain is wired).