T.I.E. Devastator
An original design, based on the question "What would Siennar design to compete with Incom's X-Wing?" The Devestator is the hypothetical bridge between Vader's T.I.E. Advanced (Episode IV) and the T.I.E. Interceptor (Episode VI) and features the Advanced model's Hyperspace drive, as well as shields, and even the ability to land on a flat surface such as a landing-pad instead of having to dock on an overhead gantry like other T.I.E. models. It even has an R2 unit for repairs and astrogation, as well as six proton torpedoes. Below is an image of the TIE Devastator sitting in a hangar of the second Death Star, awaiting Lord Vader's evaluation.
Immediately after the destruction of the first Death Star at the now-infamous Battle of Yavin, the Empire was greatly concerned about the potentency of the X-Wing's threat, understandable since it had been an X-Wing that destroyed the mighty battle-station during the battle. Starfighter designers at the Empire's primary small-ship design bureau, Siennar Fleet Systems, were asked to quickly develop a counterpart to the Rebellion's now-dreaded Incom T-65 X-Wing starfighter.
Siennar examined the inherent strengths of the T-65: it's speed, durability, and firepower, and made those the top priority for their new TIE design, code-named Devastator in an attempt at propoganda. The standard TIE fighter had its twin ion engines replaced with two massive Novaldex J-77 Event Horizon engines, which at the time were extremely revolutionary and still in the experimental stage. At this point, the "TIE" designation was no longer accurate, but Siennar retained "TIE" in the Devastator's name, primarily for marketing purposes. (Curiously enough, after Emperor Palpatine cancelled the Devastator project, the designs for the J-77 engines were sold to a third-party, and eventually were acquired by General Dodonna for use in the Rebel Alliance's own legendary starfighter, the A-Wing!)
After ensuring that the speed granted by the J-77 engines would make the Devastator at least as fast as any starfighter in the Rebellion's inventory, the weapons were the next consideration for Siennar. Mimicking the X-Wing's four Taim & Bak KX9 laser cannons, Siennar mounted four of its own equally-powerful laser-cannons on each of the Devastator's four variable-geometry wings, as well as two Krupx MG7 proton torpedo launchers housed in each wing. Each launcher fires from a three-torpedo magazine for a total payload of six warheads. This impressive armament thus made the Devastator the equal to the X-Wing in weaponry as well as speed.
In terms of durability, the TIE fighter had always been critically lacking, especially in comparison to the T-65. Heavier sedrellium armor was added to the hull, and even a modesty deflector shield system was added. Lastly, long-range capability was added by the introduction of a Class-Two hyperdrive system closely modelled after the X-Wing's, even using an R2-unit in lieu of a hyperspace Nav-Comp as the X-Wing does!
Naturally, the cost of such a vessel was outrageous for the amount of mass production that the Empire required for its daily power-projection and anti-insurgency operations, and as such was simply too expensive for final approval for production. The entire project was cancelled, especially after the cost-overruns of the second Death Star's construction began to mount.
Some of the lessons learned by Siennar during the development of the Devastator were not, however, lost. The four wing concept was retained, as were the four wing-mounted laser-cannons, these concepts as well as wing-shape and layout were then used on the successor to the ill-fated Devastator fighter, the TIE-Interceptor. Whereas the Devastator was prohibitively expensive, the TIE Interceptor was designed from the very beginning to be as cheap to produce as the Devastator would have been expensive to produce. The variable-geometry capability of the wings were also abandoned in the TIE-Interceptor, as was the deflector and hyperspace systems, the sedrellium armor, and the proton torpedo launchers. The knowledge gained by experimenting with the J-77's powerful new J-77 engines was, however, at least partially retained, resulting in the TIE Interceptor being at least considerably faster than the original TIE Fighter. The hyperdrive system, reduced in functionality by the removal of the costly R2 droid and droid-integration systems, was used later on in the production of the TIE Scout
Only six Devastators were produced in total. One was intentionally destroyed during unmanned testing, and the second one crashed due to technical reasons, although the pilot managed to eject safely. The third was sent as a "gift" to Lord Vader, as the original TIE Advanced (Prototype) fighter had been, since the Dark Lord was himself such a legendary starfighter pilot; this fighter was of course destroyed at the Battle of Endor, since it was aboard the second Death Star at the time, awaiting inspection by the Sith Lord. The remaining three were sent out to the Imperial Fleet for testing. One was lost in combat during an evaluation mission that had been discovered by Rebel spies, another was back in Siennar's possession for post-testing evaluation when Palpatine's regime was toppled at the Battle of Endor, and the sixth was still in service even after the Battle of Yavin, piloted by Imperial ace Baron Manfred Heltstrom. How Helstrom managed to acquire the Devastator is still a source of mystery.
The last quirk of fate surrounding the Devastator is that of its other successor, the more popular TIE Defender. A marginal mixture of the TIE Interceptor (itself a successor of the Devastator) and the Devastator itself, the Defender's primary goal was to somehow keep some of the main advantages of the Devastator (the heavier weaponry and hyperspace capability) while also keeping the cost-effectiveness of the Interceptor. Development costs of the Defender were much better managed than the Devastator program, so that by the time of the Battle of Endor, several of them were actually in trial-service with an active duty Imperial Squadron. Sadly, these remarkable Defenders were lost at that battle, and due to political reasons (namely the defection of the Defender's chief supporter, Admiral Zaarin) they were never produced again, although that would have been the best possible choice for the remnants of the Empire, given that the Devastator was now even more far too expensive for production.