When Bryan Singer (director of the original X-Men) began talking about his and Matthew Vaughn’s (director of X-Men: First Class) plans for the next mutant team-up film and namedropped “Days of Future Past” as its title and source material from Marvel Comics, he explained that Twentieth Century Fox had big plans to “broaden out the universe.” And by that he meant, dig deeper into the vast library of X-Men comics for big concepts that haven’t been explored in the series – or even the genre for that matter – and introduce connectivity between the films.

Singer described the project as “ambitious,” easily the most apt description of the X-Men: Days of Future Past production which not only brings together the casts of the original X-Men trilogy and quasi-prequel X-Men: First Class, but “fixes” some of the continuity errors caused by the less successful films (critically speaking) in the series. To do so will reportedly be very expensive.

The Wolverine hits home video on December 3rd and its worldwide theatrical run made over $400 million, making it the second-highest grossing film in the X-series. To put the number in perspective though against the other studios making Marvel Comics movies, The Wolverine’s theatrical take is only one-third the haul Marvel’s Iron Man 3, over $337 million less than The Amazing Spider-Man and less than what Thor: The Dark World has made in just a few weeks in theaters.

To compete, and more importantly, to stay relevant, Fox therefore must go big or go home with X-Men: Days of Future Past. They forfeited the Daredevil rights back to Marvel Studios to focus on the X-Men and Fantastic Four universes and Days of Future Past is a way to revitalize the brand and (re)launch a connected universe of their own. There’s are multiple exciting properties in various stages of development that may live or die based on the success of Days of Future Past, and with Fox moving its release date up to a more important May weekend, they’re banking hard on its success for the long-term future of the brand.

The need to deliver a bankable (and quality) blockbuster may be part of the reason we heard this week that Bryan Singer and co. are reassembling in Montreal for extensive reshoots. This normal part of post-production usually involves pick-up shots to fill out scenes and to make a few tweaks, but for X-Men: Days of Future Past, whoever they’re bringing back, it’s for two full weeks next month, meaning they’re adding more content to the feature.

Even with the film six months out after pushing its release date forward, and the fact that they’re doing major reshoots, it’s potentially a very good thing for the final product. Days of Future Past, as we continue to reiterate, has so much riding on it and such high expectations to meet, that Fox and those involved will do everything they can to make sure their vision is realized, especially considering the complexities that come with managing a massive cast with conflicting schedules. Perhaps the extra time is to shoot scenes from the script they couldn’t get certain talent together for on the same dates since scheduling principal photography was a “logistical nightmare” according to Singer.

Either way, it’s going to be quite an expense, one that the Calgary Herald claims puts it at the top of Fox’s most expensive movies ever, sitting only behind James Cameron’s Avatar – According to the New York Times, if you include marketing costs, Avatar cost half a billion dollars. According to the Wall Street Journal in an article from July, the budget of X-Men: Days of Future Past was above $200 million and it’s possible (likely, even) that it’s ballooned quite a bit beyond that (that’s also not including marketing).

X-Men: The Last Stand for instance, which was shot eight years ago, cost a reported $210 million production-wise and there’s no way X-Men: DOFP with its larger cast and shot in 3D, is less expensive. Outside of paying the film’s massive cast of stars, many of whom have Oscar nominations or wins, and constructing sets based in the future and in the ’70s, fans have yet to see where a big chunk of the money’s gone this time around – towards special effects.

We know the film finally introduces Sentinels, giant mutant-hunting robots, and combining these mechanical beasts with a large roster of characters with visually spectacular superpowers will definitely make for costly action set pieces.

The question is, is Days of Future Past more expensive than the $220 million reported budget of The Avengers or the 258 million production cost of Spider-Man 3? If it is, that would make it the most expensive superhero movie of all-time and Fox would need it to hit double the financial success of any prior film in the series. We’re living in a post-Avengers, post-Dark Knight and post-Spider-Man world, and that illusive billion dollar mark is something Fox wants to guarantee.

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The Wolverine releases hits home video December 3, 2013, X-Men: Days of Future Past on May 23, 2014. Fantastic Four opens June 19, 2015.

Follow Rob on Twitter @rob_keyes for your Marvel movie & TV news!

Sources: Calgary Herald, WSJ