What are the most innovative new OSes in active development?

> I like his ideas about tighter vertical integration between the OS and desktop apps, and a browser that is a smart document reader, as opposed to a VM for web apps.

Other actively developed OSes that fit this level of OS + desktop apps integration are:

ReactOS: https://reactos.org/

Haiku: https://www.haiku-os.org/

Redox: https://www.redox-os.org/

> Bonus points for non-POSIX plays or language choices beyond C/C++/Rust :)

Then you will pretty much like Fuchsia, a new OS by Google that is actively developed in the open and uses its own kernel called Zircon which is a microkernel optimised for modern processor architectures and multi-core machines.

Several drivers are actively written in Rust and its networking stack is written in Golang. Everything else including the kernel is written mostly in C++11.

https://fuchsia.dev/fuchsia-src/the-book

Not sure how active is it but Genode looks interesting:

https://genode.org/about/index

"The Genode OS Framework is a tool kit for building highly secure special-purpose operating systems."

"Genode is based on a recursive system structure. Each program runs in a dedicated sandbox and gets granted only those access rights and resources that are needed for its specific purpose."