What languages/skills to master to progress from senior to principal engineer?

(Edit: or, really, how to keep going once you are a senior engineer? particularly in terms of earning potential).

I have been coding for several years, in several languages -- general purpose, mostly data pipelines as well as server infrastructure. Language-wise, I did some C++ early in my career. I've actually been working with functional languages for several years hence, along with a smattering of Java, Python, etc.

I'm of the opinion that functional languages (and also Java) are not enough in and of themselves for me to continue to progress professionally at this point.

I've been thinking about next steps, and I'm considering throwing my energy into one of three directions:

1) C++ -- I've gotten the impression that doing C++ server infrastructure would allow for significant headroom in terms of earning potential (Google, finance), so I've been thinking of taking up C++ again and really becoming competent in it.

2) Machine Learning -- On the other hand, machine learning also seems to be all the rage, and that seems to center around Python, R, Matlab.... as well as stats and linear algebra.

3) Distributed Systems -- or I could dive deeper into distributed systems, which has its own set of concepts and practices to master.

Each of these seem like distinct directions to take. I only have so much time on the evenings and weekends so I want to leverage those chunks of free time well, and focus my energy in a profitable manner.

Opinions?