Ask HN: Which technology/language learn to dive into CS even more?

As a software engineer in Web, I worked with many technologies over the last 10 years, from server-side Perl/PHP/Python to frontend with all it's fancy JS ecosystem. Recently my wife started taking computer science classes in Community college, and I helped her with assembly assignments. I mean, I studied it in college myself, but all it got buried in everyday work. I really enjoyed learning more about assembly again and it's helping me at my everyday work — making my mind more open and solve problems faster.

Which languages can you recommend I should look to continue to improve myself as engineer? Better based on own expirience.