Lockheed Martin F-55
An [unconfirmed] upgrade or redevelopment of the F-35 Lightning II or Super F-35 with twin engine configuration.
Most experts agree that this would more likely be a totally new aircraft perhaps using significant parts but a scratch built design non-the-less. A little like the OG F/A-18 Hornet developing into the F/A-18 Super Hornet.
A F-22 upgrade is likely. An engine upgrade from the NGAP engine program is likely for the F-16 and F-15EX too. But a twin engine redevelopment for the F-35 into an F-55 is a hard budget ask when considering the Boeing F-47 development program in play.