My personal view is that it's too early to know what will be the no. 1 use case. It could be blogging, it could be a Twitter-like app, it could be a Patreon like app for supporting artists, it could be security of IoT devices or security of deploying cloud infrastructure, or software package manager or (most likely) we are not even thinking about the first app that goes mainstream.
The main point is that at this point in time rather than trying to pick winners we are focusing on making it super easy for developers to rapidly develop apps. They need to write less code, their apps are more secure by default, they don't need to manage a lot of infrastructure etc. This enables faster innovation and we can get to killer apps (notice plural) faster.