Demo a new proxy that can handle absolutely anything. What looks cool is spinning up a bunch of Linus VMs and pushing them through a VNC to JS emulator so that they stream direct into a browser. Where you can make money is in repurposing that proxy to solve real business problems like authentication, load balancing, traffic redirection, vanity domain support, opening encrypted tunnels, and so forth.
Deliver snacks and food to school gate from local shops combined with group buy app.
Several tens or hundreds of people are hungry or thirsty at the same time.
Said people are constantly change minds and want new and familiar experiences.
It's a unique enough proposition.
That or another 2nd hand book app.
2nd hand food app no thanks. Though I'm curious about an app that can collect refuse/rubbish which usually has resale or social value. Local councils do poorly in the app space.
Another is to find small companies that get acquihired by big software companies. What were they making that was threatening the larger company?