Some browsers support tab stacking, which allows to group tabs in a common "tab" in which those tabs become like sub-tabs, useful for ordering lots of tabs in common themes or subjects. But as it is now, Sessionbuddy records sessions with the tabs all present but ungrouped.
The browser Vivaldi (it uses the chrome engine) supports natively tab stacking, before it Opera 12 supported it also, so who knows the new Chromium based Opera will also support it.
Also Vivaldi has an option to manually save and restore a session maintaining it's stacked tab grouping, so maybe you could start from there, like analizing some Vivaldi saved session file or something.