I think a single tool to solve design + testing would be great. Cypress does Component Testing now, and you can use it in a Storybook-esque fashion, but it doesn't export to a static site for documentation, and instead of using knobs/plugins, you automate with the Cypress API. It's missing some of the design touches of Storybook, though.

Storybook is good for design, but the testing integration is definitely lacking. Having to maintain <test suite in Jest> alongside <design suite> feels like a lot of code.