I thought it might be worth mentioning here that there is currently a crowdfunding campaign to create a truly open-source and native Linux UX tool called Akira.
Since industry leading tools like Adobe UX or Sketch etc. are paid, closed source and not available on Linux that could be a real game changer for the Linux desktop (and potentially convergent Unity8).
You can learn more and hopefully support it on the Kickstarter campaign page.
Please spread the word!