The “back” functionality would close the tab, if the previous action for that tab was for it to be opened, returning the user to the tab which opened it.
This would allow a “peek” research style (I want to look at the contents of a link, but not have to reload this page when I come back), and (for people like me, who use the edge-gesture to go back) removing the feeling that I’ve screwed up the edge gesture when a link used ‘target=“_blank”’ without me noticing!
When a tab is freshly opened from a link, the “back” action would be to “close + return to <source tab>”.
The current page could fade-to-background colour (instead of revealing the previous page) to highlight that the tab is closing, rather than actually going back.
This kind of interaction already exists in the Arc Search browser on iOS.