For example, in the not-to-distant future, we won't need parking spaces. You'll just hail the self-driving car (or similar) using your phone. People won't own their own cars.
So if your startup is designed to help people get parking spaces, how much potential is it really going to have? Probably not much.
Similarly, the internet is transforming education. Information is everywhere. A startup focused on helping kids get into college feels a little shortsighted to me. It might provide a useful service to some people today in the context of present challenges, but there's no way it's going to turn into a unicorn.
Anything related to helping people find jobs is also probably a non-starter. Do you imagine a future in which we need everyone to contribute to the extent that it's worth paying them what today we would consider a "fair salary" for their labor? I certainly don't.
Take the salary part out of the job and maybe you have something. What about a platform for helping people find interesting ways to spend their time?
Take the college part out of education and all kinds of possibilities open up. For that matter, take the job training part out too. I've become pretty disgusted as Udacity, Coursera, and edX have gradually shifted toward teaching people "career skills." There are still some good MOOCs out there, but you have to wade through all the "learn these skills to make more money in your career" crap.
What about a platform to foster pure intellectual curiosity? That's what I was hoping these MOOCs would be.