X-Men: The Last Stand may be best known for ending the original run of X-Men movies and leading to a soft reboot of the franchise four years later, but in many ways it seemed to be setting up X-Men: Days of Future Past. That movie, which was released in 2014, was a direct sequel to X-Men: First Class, but it was just as much a sequel to The Last Stand.
Strangely, the clearest story thread connecting X-Men: The Last Stand to X-Men: Days of Future Past is a romantic one. The Last Stand created a love triangle between Bobby Drake (a.k.a. Iceman), Rogue, and Kitty Pryde. In Days of Future Past, Bobby and Kitty appear to be romantically involved, and Bobby also shows off the evolved version of his powers that he unlocked in his battle with Pyro at the end of X-Men: The Last Stand - transforming his entire body into ice. There are some points of confusion in the continuity, such as Professor Charles Xavier being alive and back in his old body after dying and transferring his consciousness into the body of a comatose man in The Last Stand's post-credits scene. For the most part, though, Days of Future Past works as a follow-up to the third X-Men movie.
Based on certain plot elements that were introduced in X-Men: The Last Stand, it seems as though Fox may have been planning on making X-Men: Days of Future Past sooner rather than eight years later. Here are all the ways in which the third X-Men movie set up the fifth main series entry.
X-Men: The Last Stand Featured The Sentinels
X-Men: Days of Future Past introduced a terrifying version of the X-Men's long-standing robot enemies, the Sentinels. The gripping opening battle highlights the hopelessness of trying to fight these Sentinels, which use technology based on Mystique's DNA to adapt to any mutant powers they encounter and imitate them. To make matters worse, the Sentinels share these adaptations among themselves, so Sunspot's attack on one Sentinel is used to break another free from Iceman's powers, and vice versa. In the movie's final conflict, all the X-Men can hope to do is hold the Sentinels off for long enough to give Wolverine time to fix the past - each sacrificing their lives to buy a little more time.
The Sentinels were also featured in X-Men: The Last Stand, though they weren't quite so intimidating. In fact, the real Sentinels were not featured at all; instead the X-Men simply battled them in a training simulation, which Wolverine concluded by cutting off a Sentinel's head with his claws. Though no Sentinels had been shown in the series up to that point, and the killer robot wasn't elaborated upon in dialogue, the most likely explanation is that the X-Men had gotten word about prototype Sentinels and were practising in case they ever needed to fight them. In that respect, X-Men: The Last Stand actually fits well as a prequel set about ten years prior to the events of X-Men: Days of Future Past - especially since the movie featured the Sentinels' creator.
X-Men: The Last Stand Introduced Bolivar Trask
You could be forgiven for forgetting that X-Men: The Last Stand had its own version of Bolivar Trask, the character played by Peter Dinklage in X-Men: Days of Future Past. In The Last Stand, Trask was played by Bill Duke and had a minor role as the head of Homeland Security. He took part in meetings regarding the implementation of the mutant "cure," but never mentioned the Sentinels or even gave any indication that he was a scientist like the character in the comics. However, the inclusion of both Trask and the Sentinels in a limited capacity doesn't seem accidental. It's likely that X-Men: The Last Stand was setting up both to play a bigger role in a future movie - which they both eventually did.
There's little physical resemblance between Duke and Dinklage, but X-Men: Days of Future Past wasn't the first or last time that the X-Men franchise introduced a different version of a character that had already been featured in a previous movie. Dinklage's version of Bolivar Trask was a lot closer to the version in the comics. He was the scientist who had originally discovered the X-Gene that gives mutants their powers, and used his knowledge of mutant DNA to create the Sentinels that would come to almost wipe out mutants in the original timeline.
X-Men: The Last Stand Moved Mutant Hate Forward
Perhaps the most important seed planted by X-Men: The Last Stand was how the oppression of mutants had escalated to the brink of all-out war. The mutant "cure," originally billed as an optional treatment for mutants who had grown sick of their powers (or, more likely, sick of how they were treated because of their powers), was soon revealed to have been weaponized. After William Stryker tried and failed to kill every mutant on the planet in X2: X-Men United, the cure was devised as a softer approach to eradicating mutants. However, the end of the movie revealed that the effects of the cure were not permanent, as a recovering Magneto was able to use his powers to slightly move a metal chess piece.
It makes sense that the introduction of a cure for the mutant gene would have shifted the X-Men universe's Overton window from general fear and hatred of mutants towards an acceptance that the mutant menace needed to be destroyed completely - especially after Magneto very publicly ripped the Golden Gate Bridge apart in his attack on Alcatraz. The President of the United States in X-Men: The Last Stand was noted as being relatively mutant-friendly, so after a major terrorist attack he may have been beaten in the next election by an opponent who promised to crack down on mutants.
X-Men: The Last Stand is regarded as a low point in the X-Men franchise, but there are elements of the movie that serve as important set-up for X-Men: Days of Future Past. While we wait to find out what the future of the X-Men will be at Disney, it's definitely worth watching the two movies back-to-back to see how that set-up was paid off.