I find real-time audio mixers super interesting. A lot about writing your own can seem easy, until you find yourself in a fight against latency. If you hit 30ms you're dead, the mixer will become almost useless in a live setting. That's why the big expensive equipment hasn't been replaced by iPads. The highly regarded commercial SQ mixers [0] have 0.7ms end-to-end latency thanks to an FPGA inside.
I also firmly believe that, in the future, everything will be connected via Dante [1], which will be another example of commodity(-ish) computer networks eating everything.