For example, Uber appears to be the end of the taxi business, but it opens up a whole market to developers. You have a beautiful imbalance now, all those taxi drivers and cab companies are heavily incentivized to compete with Uber or lose their companies/jobs.
If there was (and there might be, idk) a platform that allowed all the cab companies to aggregate into an uber-Uber, there's a market for that with known customers. The only obvious issue I see is that in order to make this work, the cab companies would have to give up their fights to block Uber using municipal codes as they are now.
Anywhere you have an imbalance, there's a potential market. The problem is, if you're not on the demand side, your product has the potential of a short lifespan unless it corrects the imbalance.
Google did this with ads, Uber with rides, AirBnB with rooms, tinder with.. well, you know.
Each of these markets had an unseen demand side which each of the companies exploited. Find an unfilled demand and fill it. It doesn't need to be huge, but if it's in the way of someone already moving there, you greatly increase your chances of being acquired.