Sounds like someone's looking for a new project to work on :)

Though this doesn't answer your question directly, I would say that a really great idea for any app should always be kept secret. You can learn more about why secrets are important when it comes to innovating and building companies in Peter Thiel's book Zero to One: https://www.amazon.com/Zero-One-Notes-Startups-Future/dp/0804139296/ref=sr_1_3?ie=UTF8&qid=1531886547&sr=8-3&keywords=peter+thiel

To answer your question though, I think there are a lot of ideas out there that are up for grabs. Blockchain technology is still so new, and if you think about blockchain's chief benefit (decentralization), you will see that the nature of it is to replace existing products and services. For example, Musing is a replacement for all the question and answer websites done in the past. There are plenty of different products that could be built as a decentralized system on Steem as well. I think that something like TripAdvisor on Steem could be pretty cool. Personally, I also think that a news aggregator like Reddit could also be useful, since Steemit isn't really a link-sharing website as much as it is a blogging one. These are just examples, and I'm sure you can think of even better ones that would be a fit for the Steem blockchain :)