No need to fork Brave, we will collaborate on extensions APIs as we scale. We're supporting tested chromium extensions that we curate and fetch from our own S3 (essentially a cache backed by the Chrome Web Store).
For an extension to get support, we need to rank it as in-demand, and then try adding it to Brave to see what missing chrome.* APIs it might need. https://twitter.com/bravesampson is leading this charge, using https://github.com/brave/browser-laptop/projects/1 to keep track and engage with developers.
For future extension APIs that go beyond the chromium ones, we won't support XUL, but I'd like to take advantage of our Muon (https://github.com/brave/muon - we forked Electron, because security) React.js-based front end.
In 2Q2017 I'll hope to have more to say about extensions and where we'll go. Interested extension developers are welcome now to join our https://community.brave.com/ discourse. Thanks.