Extremely low latency would help for RTOS devices. Machine control like CNC milling machines and lathes and engravers. Software defined radio and other DSP topics. Aside from the previously mentioned HFT suggestion.
As a profitable application, consider the worlds best gcode driver with extremely advanced lookahead and real time guarantees. Just pump gcode in a serial port and drive servos out a parallel port as fast and smoothly as possible. The problem with selling an app is you might have built an foundation for an interesting class of apps, but you'd need someone with domain specific experience to write the app.
As a profitable application, consider the worlds best gcode driver with extremely advanced lookahead and real time guarantees. Just pump gcode in a serial port and drive servos out a parallel port as fast and smoothly as possible. The problem with selling an app is you might have built an foundation for an interesting class of apps, but you'd need someone with domain specific experience to write the app.