I did this too. 5 hour stream, started with google docs, trello then figma before building. Lot of fun and having the camera rolling kept me focused instead of getting distracted by twitter or notifications.
I decided to build something for nonprofits:
I found a number of charities that directly equate an action (do x things for y people) with a dollar amount. I pulled together a list of the charitable actions and chose a base product that people were familiar with (the cost of a pumpkin spice latte). Then I sorted them by how many lattes you would have to skip to be able to donate to compelete the charitable action.
It was a lot of fun and I hope to do something similar again.