F-106X (Model 8-28/8-29)
The F-106X was a 1956 design study for a Delta Dart follow-on.  With the XF-108 effectively dead, Convair attempted to modernize the F-106 design. This study envisaged an interceptor with a canard layout that was powered by a JT4B-22 turbojet fed by rectangular air intakes. It was envisaged as an alternative to the Lockheed YF-12 (later SR-71), and was to have had a fire control system with "look-down, shoot-down" capability fed by a 40-inch radar dish.

The F-106X was extremely advanced for its time with Mach 5 performance envisaged by Convair. Carried over from the F-106, the only thing would have been the basic delta design. A more powerful engine was to be mounted with redesigned intakes to account for the new engine. Canards were added on the intakes and the cockpit was raised to improve visibility. The fire control systems were to be given a complete overhaul with the AN/ASG-18 previously developed for the XF-108.

Under the F-106X development project were the designated F-106C/D aircraft, with "C" being the single-seat version, the "D" being the two-seat version. At one time the Air Force had considered acquiring 350 of these advanced interceptors with several attempts initiated to upgrade the entire current F-106 fleet, but none were approve by Sec. of Defense Robert McNamara and the project was cancelled on 23 September 1958. Later Sec. McNamara pushed through the F-4 program instead. 

F-106C/D Super Dart
The F-106C/D project got as far as mounting a new nose on an F-106 to test the practicality of the design before the Model 8-28/29 project was cancelled. Two production F-106A's; 57-0239 and 57-0240 were modified to test that new radar housing, which was a five-foot nose extension. Only 57-0239 actually flew making 10 flights with this new nose in 1959. The plane was later destroyed in fatigue tests. 57-0240 eventually reverted back to standard F-106A configuration, but never flew with the modified nose configuration.

F-106 Sky Scorcher

In a separate effort to the Convair Model 8-28/8-29 of the F-106X project in 1956, was another advanced F-106X concept project called the Convair Advanced Sky Scorcher F-106, which never officially had a model number assigned.

It was proposed to support the Sky Scorcher 'Mass Raid Weapon' Missile Project proposed by the Convair Division of General Dynamics to the USAF in 1956. Sky Scorcher was a very large missile, which was proposed to be capable of carrying a thermonuclear warhead with a yield of two megatons. The oversized warhead would be used against attacking formations of supersonic bombers; it was anticipated that fourteen such initiations, at a distance of approximately 460 miles (740 km) from the bombers' target, would be sufficient to disrupt an attack. Proposed to carry this missile was a force of 80 of the enlarged version Convair Advanced F-106 Delta Dart interceptor, which had, at the time, not yet entered flight testing even in its baseline form. The Advanced F-106 would have a modified weapons bay to carry the Sky Scorcher missile and an infra-red seeker at the top of the vertical fin. Despite the Convair sales pitch and the anticipated effectiveness of the weapon, the Air Force was unenthusiastic about the concept; aside from the expense of developing the aircraft and weapon. The Sky Scorcher missile also suffered from the fact that there would be significant effects on the ground below the location of an air-burst of a multi-megaton nuclear warhead. As a result, the project was abandoned before any significant work was undertaken. During all this however, Convair continued with design and wind tunnel tests for this Advanced F-106, which they also dubbed Sky Scorcher. They performed Inlet Wind Tunnel Tests from 1957 and 1958 in the NASA wind tunnels as seen at http://crgis.ndc.nasa.gov/historic/1251_Models_D-G#F-106_Inlet

F-106E/F
On 9 February 1968, the Defense Department announced they were not going to purchase the Lockheed F-12A interceptor (later the SR-71), opting instead to remain with the F-106 as the primary interceptor to protect the continental USA from air attack. 

On 3 September 1968, Convair issued a proposal for an "improved" interceptor that was to be designated F-106E/F. It was to be compatible with the upcoming airborne warning and control systems as well as with the over the horizon radar defense network. The F-106E/F would have had a longer lose, with a new and improved radar with a "look-down/shoot-down" tracking and missile launch capability. It would also have had a two-way UHF voice and datalink radio. It would be capable of launching both nuclear and non-nuclear missiles, including the AIM-26 Nuclear Falcon and the AIM-47. Unfortunately for Convair, this project never got off the drawing board.

F-106J (Japan Version)
The Delta Dart was never exported to foreign air forces.

There was however, a proposal for an F-106 version for Japan (F-106J) with an MG-10 fire control system (the same one that was fitted to the F-102A Delta Dagger) and six Super Falcon missiles.

It was also to have ground-attack capability, with a pair of pylons underneath each wing capable of carrying bombs or fuel tanks. The Japanese sale never took place and several years later Japan undertook manufacture of the F-4EJ Phantom.

Nothing ever came of this proposal

Other
A pair of F-106's were displayed at the 25th Paris Air Show in June of 1963 to help entice other countries including Japan, but no customers were forthcoming.

Convair tried to interest Canada in a Canadian version-not merely as in interceptor but also for the strike role.

Nothing ever came of this idea. There were also plans for F-106 final assembly and production in Germany, but these plans never reached fruition.