Usually the planewanks are connected with planes that did not enter service at all or did not enter service in particular country. What about the other way?
Here's some candidates:
USN:
F-14: This ugly plane would be unnecessary if F-4 was modernized, fitted with better radar, lower fuel consumption engines and AIM-54. This kind of Super Phantom could have served USN magnificently and might have even gained a number of foreign sales.
Sweden:
Viggen: The improvement potential of Draken was not fully used.
Gripen: Fine plane, but way too costly even for a scenario in which Cold War was to continue. F-18 (as proposed OTL) would serve Flygvapnet just fine.
RAF:
Hawker Siddeley Nimrod: Excellent ASW aircraft, but as the numbers were low one is hard pressed to find just why not produce Canadair's CL-28 based on British airframe and already made instead? Other candidate might be P-3 on licence. About current MRA4 disaster one does not need to mention.
SEPECAT Jaquar: I've always wondered what was the business logic for this?