Most of those take a lot more time and money than YC usually offers. There are some opportunities in "New Defense Technology". Something like a low-cost replacement for the Javelin anti-tank missile based on off the shelf phone camera parts ought to be possible. Of course, once that's out there, every insurgent group will have some. "Explainable AI" is really important. "Stablecoin finance" is mostly how to make sure…
Phone camera parts would be overkill. The Javelin sensor isn't nearly that high-resolution, we're talking low triple digits in "pixels". It does however refresh its readings very fast, a necessity given its speed. The old Javelin used active cryo and a filtered IR imager, the new one is passive like the IR camera in some phone attachments. It is stupidly simple in operation: CLU provides the target "signature" and the imager seeks it. After the initial ascent in top-down mode, a stronger signal on one edge of the sensor pushes the control surfaces in the opposite direction until it strikes its target. I'd give the Wikipedia page a read. It contains a surprising amount of information that informs the design and thought behind the missile. Military systems are cool for how robust yet simple they are.