I completed my OMSCS last Spring, specializing in Machine Learning. My advice to you would be to not worry about any specialization as you can change it later on. Focus on taking courses that interest you. You need to complete 10 courses for completing the program. With 2-3 mandatory courses + 3 electives, you have room for at least 4 other courses not in your specialization.

There are some interesting courses on the Computing System track. FWIW, my undergrad was in a different engineering field. So I really enjoyed the Network Security and Intro to Operating Systems. The Algorithms course reshaped my perspective on formal methods and correctness. The HCI course is also very good.

As mentioned by @bewestphal , check out https://omscentral.com/ for reviews to get an idea about the popularity, difficulty and time investment for the courses.