We've been polling the add-on developer community what APIs they need, and also we have our own ideas. Bill already has some ideas collected here: https://wiki.mozilla.org/WebExtensions/Future and here: https://wiki.mozilla.org/WebExtensions#Additional_APIs, the intention of this bug is to collect any features that we might want to implement and do not belong to webextensions-chrome-gaps.
Please, could anybody add "Depends on bug 1332447"? I can't edit the bug.
I think what Bug 1246236 should be added to dependencies.