I interned at a company which makes Chromium mods tailored for a specific market (not Vivaldi), so AMA.
In short what we did is pretty much the same thing as other Chromium mods: take the Chromium source and add a bunch of features on top of it. Some features that I could remember:
* English dictionary / translator (our market's native language is not English)
* Video downloader (includes Youtube)
* Torrent client
* Multithreaded downloading
* Facebook unblocker. The company operates a bunch of proxy servers, and the browser can be remotely configured to proxy certain sites through those servers. Handy when the government decides to block Facebook/Medium/...