Built-in GUI customization for changing tabs orientation I find it very useful since most people are using 27 inch + nowadays with enough room to spare for any webpage and it makes navigation through multiple tabs easier. As of now third party plugins would only add additional vertical section leaving the top bar tabs visible and it defeats the purpose of having it.
Very nice browser native implementation of this feature you could find in Vivaldi web browser for example.
I hope to have this feature soon in opera itself. it would be awesome.