Plasma is a physics-based engineering playground sandbox title that will make full use of the Steam Workshop. You'll be able to create "mechanically complex devices" using a physics engine that's state-of-the-art, along with visual programming tools that are user-friendly. As for what you'll be able to design, that can range from arcade machines through to complex robots, spiders, and even entire game worlds that can then be shared with the community.
For example, if you see something that you really like, you'll be able to download it and iterate upon it. If you like how a certain car looks and want to know how it works, you'll be able to figure it out and then change it however you want.
Plasma is set to launch on Steam sometime in 2023.