Networks. Compiler theory. Language design. Web APIs. Caching layers. Core algorithms. Packet switch microcontrollers. FPGAs. CRUD apps. Big Data. Anti-abuse tooling. Application security. Machine learning. Any topic you possibly want to throw an engineer at, an investment bank is already throwing a team of engineers at.
Our Chapter 1 release includes levels which have you jailbreak multiple chips (AVR microcontrollers) on a custom handheld device given to minimally trusted gophers to own up a stock exchange through their REST API. And that isn't even a theoretical attack.
Additional factors which tipped the scales include "finance has a default goal function which is really easy to understand", "some subsets of financial work are surprisingly easy to model" (like a stock exchange is), and "finance has narrative choices which allow us to excite people without getting typecast as A Video Game For People Who Played Dungeons and Dragons In High School" (which is, for a variety of reasons, important to the team, including to me, the ex-DM).
Other strong contenders for our first game were "Like Dwarf Fortress but with code" (harder to build than a stock exchange; includes the word "dwarf") and "a nuclear power plant" (slight problem: we know nothing about nuclear power plants).