(mostly copied/pasted from my findings at T242918#5824735):

We were generating a gadget variant before, and it seemed to work, so I think there's no immediate technical barrier to restoring it.

The system could mimic MoreMenu or HotCat, where the code lives in one place on Meta and the applicable wikis will import it into their local gadget. Translations live as separate JS files, and the appropriate one is loaded based on user's interface language, falling back to English. In the Grunt build step for the gadget, we'd generate these JS files from the JSON similar to what we're doing now. We can use a simple Node script to sync the files with the wiki.

Possible issues:

• The gadget could in theory be forked and the "global" version maintained by us isn't what is seen on the relevant wiki, but I don't think we have any way to prevent this, nor should we worry about it. Other long-standing gadgets use this same system without issue. What usually seems to happen is someone will look at say English Wikipedia's gadget (or wherever they discovered the gadget) and copy it to their wiki. In this case the code would be something like mw.loader.load('https://meta.wikimedia.org/w/load.php?modules=ext.gadget.WhoWroteThat');, so they would have actually done it correctly (though of course it may not work for their language).
• The code that lives on-wiki will be the transpiled version of the ES6+ code, meaning it might be hard to read. I don't think this is a huge risk either, since we can write in the comments to refer to the original source code for reviewing and proposing changes (this is what is done for MoreMenu).
• Requires manual deployment, but this should be much more simple than deploying the browser extension.
• Code-wise, there are just a few things we need to fix that don't work in IE 11, if we care to fix them. Safari might have some problems as well. I don't think we need to spend considerable time on browser support, as long as we're clear in the documentation on what browsers we've tested.
• Translations may be made on-wiki when they should happen on translatewiki. For this, I recommend adding a comment in the source, "Please use translatewiki to add translations: https://translatewiki.net/wiki/Special:MessageGroupStats/whowrotethat ". You can use babel-plugin-add-header-comment in our build step to automatically put commentary at the top of every file (or specific files).

If possible, develop an experimental gadget (usable by the team for testing purposes)

We came up with this checklist for what will be involved. I've added this to the task description.