Big-data, AI, HFT ... Basically anything that has been and is a hot topic and which generates a lot of revenue. But I am clearly not thinking of an average Joe level of expertise. It requires to have all around the stack skills including the ability to grasp the big picture of the product and drive it forward.

Technical skills wise, for areas from above, you'd usually be looking at the strong background in algorithms, math & statistics, internals of operating systems, underlying hardware but mostly about the CPU at the microarchitectural level and strong expertise in at least one programming language.