Just wanted to add, that all what was implemented in Arc is good to have. It just missed the tree tabs support for regular tabs, which we already have there.
So, nested folders for pinned tabs and tree for free\regular tabs is way to go.
Renaming for pinned tabs is must have, as they are actually bookmarks, not real tabs. But as actual tab title has dynamic nature, and changed from page to page, it is good to have some placeholder in name, where we what to put actual page title. Like %s is now implemented in Orion bookmarks for url. 
I would like to add prefixes for my working project tabs, for easy recognition, but won't lost what page is actually is open in this pinned tab. For example, tab name: "[QA]: %title%" is expanded to "[QA]: Project Name - Page title".