With the arrival of Tab Islands, I think it's a good time for Opera to join the Vertical Tabs. Opera has always been very usable and I think it is the feature that is missing, since it is more than proven that the grouping of tabs together with Workspaces is a very useful feature.
On the side we could have the current bar and add an editable space just to your right to add the Vertical Tabs with Tab Islands to group the tabs (with or without indent), taking advantage to give more customization to the Tab Islands (color, title,...)