F-2 is practically a Japanese version of the “Agile Falcon” proposal which was an enlarged F-16 with bigger wings. Quite much like what “Super Hornet” is to F/A-18 Hornet, though motivation for the change was different.

Super Hornet grew mostly to gain more range (and also got a smaller RCS), Falcon was mostly going to grow bigger wings to keep the wing loading low when the plane had already gotten much heavier to regain the lost manoeuvrability from the extra weight. However, the bigger wings also gave bigger fuel tanks and more range.

The wing shape of F-2 and the “Agile Falcon” proposal are identical.

But just like with Super Hornet, in the end over 90% of the structural parts ended up being different, meaning that the “economy of scale” advantage would have been lost.

Agile Falcon never went to mass production in USA, as the cost increase over normal F-16 was considered too big, and Agile Falcon would not have been the “low cost fighter” F-16 was meant to be anymore. Instead, improvements to the “old small-wing” F-16 frame was continued, new F-16’s with the original “small” wings and frame but new engines and new electronics were made. And now some of those newest F-16’s have more advanced engines and avionics than the F-2’s.

But Japan wanted the improvements Agile Falcon was offering, and they did not fully understood how much those would cost in the end, so they finished the development of the Agile Falcon as their F-2, and then ended up paying many times more for their F-2’s than an equally old F-16 model with same level of engines and electronics would have cost.

In practice F-2 is still using same engines (F110–129) than some F-16 variants, so in practice the commonality is over 10%, though in F-2 the engines are license-built in Japan by IHI corporation, not by GE in USA.