With all this hype over how Snapchat refused a $3 billion acquisition offer from Facebook, it's worth pointing out that there are still plenty of ideas floating around that could score you an acquisition offer from a major company like Google or Facebook.
So, want to get acquired by Google for billions of dollars? Here's an idea to get you started:
Finding the best route between locations still hasn't been solved. It's true, Google Maps does a good job - but Google's definitely willing to pay to improve it, especially with competitors like Apple Maps on the scene.
How could you improve Google Maps navigation? Give users more control over preferences - and give them more options and combinations. Let me tell the map program that I'd like to get from Pittsburgh, PA to New York City in the cheapest way possible, then have it propose flights and buses. Not only that, have it be intelligent about it. If I take a flight from PIT airport, it should know that I'll need to get there somehow, and provide the options of taking the airport shuttle or getting a taxi.
We're long past the technological point where we should have to coordinate our travel on multiple websites - why, then, does Google Maps not integrate with forms of transportation besides mass transit and driving? Specifically, flights, trains, taxis, and intercity buses all need to be included before it can really become a single hub for all transportation needs.
So, what're you waiting for? Go make this and get acquired by Google! Or, if you have any questions or ideas, feel free to share them in the comments.
As pointed out by /u/printf_the_musical on reddit, a good place to start would be to look at Google's General Transit Feed Specification, which enables mass transit organizations to share their route data in a usable way.