What app do you think is missing from the Steem blockchain?

We already have some great apps on the Steem blockchain, the list is endless, but what do you think is missing? Which app would you love to see on the Steem blockchain which isn't already there?

Note: I realise that some people might not want to share their great ideas, but just remember that having an idea is only a small part about making a great product and it's more about execution.