Landing and reusing the first stage booster. This is why Vulcan is designed to only recover the engines, because recovering the whole booster while still launching a useful payload was thought to be impossible. It was thought that the stresses of re-entry would be too severe without excessive weight increases and that it would not be possible to retain enough fuel for a propulsive landing.
These things have existed for a century now. The fact that no one tried and it took a person with money from early dot com start up exit means others goofed up enormously.
Anybody could have tried. They didn't. Pulling down other peoples achievements because one didn't try the obvious themselves already reeks of envy.
