Twentieth Century Fox debuted the third and final full-length trailer for X-Men: Days of Future Past yesterday evening during Conan O’Brien’s late night show and it offered the best – and most exciting – look at the mutant team-up film yet. It’s hard to believe, but we’re just over a month away from the release of the next X-Men project, the biggest and most important film in the franchise. It’s so important in fact, that Fox is working with other studios to help promote it in an exciting Marvel-related way…

Fox is banking hard on their X-Men brand, and are beginning to realize the full franchise potential of the coveted licensed property. This is part of  the reason producer Matthew Vaughn (who was originally set to direct Days of Future Past), director Bryan Singer (who started the franchise back in 2000) and writer Simon Kinberg decided to make a sequel to 2011’s X-Men: First Class that introduces time travel to “broaden out the universe,” and take advantage of the decades of lore, characters and stories the Marvel Comics offer.

On the film side, while X-Men was first out the gate (first to hit theaters) compared to Sony’s lucrative Spider-Man series and Marvel Studios’ Avengers universe, Fox’s Marvel-based theatrical releases haven’t seen the same level of success at the box office, nor have they been as consistent on the critical reviews front. That’s what makes Days of Future Past so important and that’s why it’s not surprising to see reports labeling X-Men: Days of Future Past as the most expensive superhero film of all-time, and the second most costly production for the studio – sitting only behind James Cameron’s Avatar.

Fox needs X-Men to be a dominant force at the box office, so they can use the momentum to push spinoffs and crossovers. After giving up on Daredevil, another property the studio owned, and letting the rights to it expire and revert back to Marvel, Fox is doubling down on the X-Men and their other well-known team of heroes, the Fantastic Four – a franchise that’ll be rebooted next summer by director Josh Trank (Chronicle).

The studio is so confident in the next X-Men and FF movies, that they not only signed a three-year deal with writer Simon Kinberg to help write, produce and plan stories that span multiple films, but they just recently officially announced release dates for the Days of Future Past followup, X-Men: Apocalypse, alongside Fantastic Four 2, Wolverine 3 and another unannounced X-Men movie in 2018. Inevitably, these films are leading to a Fantastic Four-X-Men crossover, something Mark Millar (Fox’s comic book movie consultant) has been teasing ever since bring recruited by Fox.

But that’s only one level of crossover potential – an increasingly hot button topic fans, media and studio executives frequently bring up in discussing or promoting these films. While Fox just revealed their own slate of upcoming superhero films that will ultimately share a unified continuity, they did so a few months after Sony announced and confirmed a series of Amazing Spider-Man followups that include Sinister Six and Venom. And then there’s of course the main Marvel Cinematic Universe from the Disney-owned Marvel Studios which we last heard is planned out all the way up to the year 2028.

Each studio has already, or is just about to, craft Marvel-based cinematic universes. But the problem is that there are three of them, all entirely separate – even though in Marvel Comics, it’s all under the same umbrella. That doesn’t mean it’s always going to be that way though…

If you speak with X-Men franchise producer Lauren Shuler Donner, Marvel Studios president of production Kevin Feige, Fox consultant Mark Millar, or even Wolverine himself (Hugh Jackman)- all of these important figures see potential in these brands crossing over in some capacity down the road, but there’s no need to rush – these things are all so new, and there’s plenty of character introductions and universe building that needs to come from all three studios first. But, it almost happened in 2012 with The Avengers.

Towards the end of the production of Joss Whedon’s The Avengers, Sony and Disney-Marvel almost worked out a deal which would see the team-up movie help advertise Marc Webb’s The Amazing Spider-Man reboot. The Oscorp tower from the new Spidey universe was almost going to appear in the same New York City skyline as the Stark Tower in The Avengers but it was too late in the process and for a variety of reasons, it never happened. But it at least showed that the studios were willing to share ideas, get creative and start talks of these sorts of things happening. In the end, all of these properties share the same red Marvel banner, right?

For that reason, it should’t be too surprising that fans who check out The Amazing Spider-Man 2 in theaters in the first weekend of May and stick around through the credits, will get to see a little piece of X-Men: Days of Future Past. It’s a tradition now that all Marvel movies feature button scenes during and/or after the credits to tease something from another upcoming film or offer a little fun moment for fans, but this is the first time we’ve seen a studio film use the opportunity to tease another Marvel-branded film from another studio.

The scene in question features Mystique (Jennifer Lawrence), Havok (Lucas Till) and William Stryker (Josh Helman) in a Vietnam camp (there’s a hint of this in the latest X-Men: Days of Future Past trailers) during a neat little action sequence. It’s important to note that this isn’t any sort of crossover scene or tease of the Spider-Man and X-Men universes sharing the same space. It’s simply an exclusive trailer/teaser of sorts of another movie, but at the end of the film instead of during the normal previews.

And it’s not the content that’s important in this case, it’s the fact that the high-ups, decision-makers and marketers of the two studios (Fox and Sony) have come together to cross-promote, and the timing couldn’t be better since X-Men: Days of Future Past opens in theaters just three weeks after The Amazing Spider-Man 2. Any chance that the new X-Men will tease Guardians of the Galaxy in a similar manner?

This isn’t the first time Spider-Man and the X-Men have shared some fun-loving crossover action. Hidden as an Easter Egg on the original X-Men DVD, users could access a blooper scene where Spider-Man tries to steal the spotlight from the black leather-clad X-Men lead by Cyclops:

This is just the beginning of something that’ll keep growing. Excited to see studios begin working together in this capacity? In the long-term is it inevitable that the studios will crossover their properties?

[Update: Variety has some insight as to the how and why this particular marketing deal came about. Marc Webb had a contractual obligation to Fox after signing a deal with them after 500 Days of Summer and in order to let Webb direct The Amazing Spider-Man 2 for Sony, Fox let it happen in exchange for a bit of free marketing for their X-Men movie. That translated into this button scene.]

More: Marvel Crossover Movies Likely But ‘Not Anytime Soon’

The Amazing Spider-Man 2 opens in theaters on May 2nd, 2014, followed by The Amazing Spider-Man 3 on June 10th, 2016, and then another Spider-Man project (possibly Sinister Six) on May 4th, 2018.

X-Men: Days of Future Past arrives in U.S. theaters on May 23rd, 2014, followed by Fantastic Four on June 19th, 2015, X-Men: Apocalypse on May 27th, 2016, The Wolverine 3 (not the official title) on March 3rd, 2017, Fantastic Four 2 on July 14th, 2017, and an unspecified Marvel film on July 13th, 2018.

Follow Rob on Twitter @rob_keyes for your Marvel movie news.

Sources: Empire, Sony