Level progression would be driven by beating levels and gaining resources for opening up more levels. The overworld would contain puzzles of its own, so you wouldn't have to beat every level to proceed, you could gather the remainder of the resources in the overworld, allowing you to bypass levels you don't like.
Each section of the overworld would end with a boss that goes with, and explains the previous levels' theme. Beating the end boss would open up the next section.
Other things we've thought about were giving players the ability to color their characters by finding special items in the overworld, adding competitive elements to multiplayer, and opening up bonus levels.
This system was inspired by the N64 era games like Super Mario 64. We'll show you more once we've progressed enough with development.
In other news, the Nintendo Switch has arrived, and it appears to be successful. We like the console's idea, and enjoy playing on it, so we've decided to keep the Joy-cons in mind when designing the game. We've even managed to make them work on our PCs so we can test the game with them!