Cool things to build
A good problem solving machine
A police radar that takes a live photo or series of photos while using radar to get evidence of speeding etc.
Headphones for use at shops that act as walkie-talkies, ear protection and connecting to machine faults and users phones, smart watches etc as needed.
A standard for data passing. Like taking a bluetooth signal, passing over wifi to cellular to another wifi to another end bluetooth device and back.