The F-20 was supposed to be the successor to the F-5E Tiger II. And it could've been one of the USAF's frontline tactical aircraft. But the F-16 sidelined the F-20 and, w/o potential customers, the aircraft program was later scrapped.
What if Northrop decided on different approach? Instead of (possibly) competing with the F-16 for sales, Northrop offered (and later, possibly sold) the F-20 exclusively to existing users of the F-5E, or even the older F-5A?