What could of been with the F-20 Tigershark.
At the end of the day, a tarted-up F-5 is still a tarted-up F-5. The F404 has better fuel specific fuel consumption, but that gets eaten up if the extra thrust is taken advantage of.
The F-20 gets a larger horizontal stabilizer, a larger LERX, and a new radar but the rest of the F-5's limitations are present - low fuel capacity/endurance, limited wing clearance for weapons loading, radar limited by the size of the antennae dictated by the nose dimensions, and 1960's aerodynamics. Some of which might have been mitigated by making more substantial changes, like the way the J-7E and later replaced the original MiG-21's delta wing with a cranked delta with better low-speed handling and way more internal-tankage volume.
Northrop's real step forward was the YF-17, and the Tigershark was a poor consolation prize after McDonnell Douglas took over the Hornet project.
The thing that's always puzzled me is the $1.2 billion USD pricetag attached to the project (back when that was actually a lot of money). It sure didn't seem to go into new engineering, so I wonder how much ended up as marketing/sales/lobbying spend and how much was maybe disguised spending from (literal) stealth projects like Tacit Blue, B-2, and YF-23.