Another idea:
Subscription channels. You subscribe to a set of articles ( category, or "this article and all articles that link it"). if you are on-line, a robot slowly mirror these categories as static files on the hard disk. (update if changes are made, download if there are new). Make so this extension open a sidebar, where you can browse your offline categories, and open one of these, make so If you click on a link that is not mirrored, a popup ask "This article is not saved offline, do you want to add to this category?".