The team behind the modular OpenUSD editor ShapeFX Loki released its Apache-2.0 licenced framework: OpenDCC.

From the README:

OpenDCC is an Apache 2.0 licensed, open-source Digital Content Creation (DCC) application framework for building modular, production-grade 3D tools. It combines a flexible, plugin-driven architecture with an industry-standard Qt-based interface, embedded Python scripting, OpenUSD and Hydra integration.

It looks very promising and I'm waiting for the first packages to try it.