Along with nested vertical tabs, I'd like to suggest folders or groups for vertical tabs.
The ability to drag and re-organize vertical tabs is very helpful. However, I find it doesn't necessarily always make sense to organize tabs under other tabs.
What I would like is to be able to name a tab group, e.g. "Social Media", and then drag and drop relevant tabs under this heading.
This is very similar to the extant functionlity of being able to drag and drop tabs into nested hierarchies, but adds the ability to add a name that is not a tab itself.
These could be implemented as folders that could themselves be nested. I.e., within "Social Media" you could have tabs as well as another folder.