I think the ability to have groups be on the sidebar, but tabs on the topbar, would be a really good feature. I love groups, but I find myself often still just using multiple windows instead of the groups. When I learned about vertical tabs, my brain instantly went "that would be great for groups". I could fully hide them when I don't need them, and when I do, it'd be very quick to switch between them, and quite a bit less jarring than using separate windows. Plus, they'd be named, so no more alt-tabbing through a rotation of windows searching for one, and it'd make reorganising tabs easier. Honestly, even just allowing for both vertical and horizontal tabs at the same time would be nice.