Great answers so far! Somewhere between Computation Architectures and AI / ML. I think getting a solid foundation in Distributed Systems really pays off. There are plenty of grad level courses online. That read and discuss papers on BigTable, Spanner, MapReduce.

But a more practical example would be to watch some of the AWS:Ignite videos on the Netflix stack. And try to implement a basic clone of Netflix. Thereby getting hands on experience with routing, load balancing, ec2, s3, red shift, solr, lamdba, and so on.

Good luck!