It would be really great if Opera added a Notes feature to the browser. There are plenty of extensions out there, but I much prefer taking notes via the browser than trusting content to extensions.
It looks like there used to be a feature like this on an older version of Opera, which makes me hope there will be a return to that functionality again. Here are a few features I'd love to see:
You can do a search for something highlighted on a page with the context menu. Just choose the search engine you want from "Search With". That does not work well with StartPage or Qwant, possibly because they use a different way to load search terms.
There is V7 Notes extension which is pretty good, but has it's mishaps. Built in would be better.