Since Edgar Wright jumped off Ant-Man, we suspected Marvel would look in-house to find a replacement. And since Marvel knows the system, they know that Wright was beloved, and a replacement would have to please the comic-geek contingent who will help drive the excitement for a niche property. But did anyone think that they would just look to one of their other movies?
UPDATE: The page referenced below has since been pulled down, meaning the "source" was unreliable, or Marvel doesn't want their news to break in such a fashion. Make of this what you will. Now, from earlier...
FROM EARLIER: Comic Book Movie claims that Marvel's replacement on Ant-Man will be James Gunn, the director of Guardians Of The Galaxy. Which sort of makes sense, as Gunn has the chaotic comedy-genre chops to handle such a concept. Gunn started out in the world of low-budget Troma, so his attitude is certainly one of a "just get it done cheaply" nature. If Ant-Man production isn't working out, Gunn's going to be quick to steamroll through in a way Edgar Wright likely wouldn't have.
If true, this suggests Marvel is gung-ho about Guardians Of The Galaxy. Which wasn't the case in recent reports, which suggested that Guardians was being seen as too "out-there," necessitating a more conservative approach on Ant-Man. Of course, those are just journalists drawing connections with silly straws – the promotional campaign for Guardians Of The Galaxy has been a fairly muscular one, labeling the film as the late-summer hit everyone's gotta see. The fact that there are currently no plans for a sequel have to do with the unreliability of the Guardians brand itself: hiring James Gunn would be a show of support for both Ant-Man and the coming Guardians Of The Galaxy. Unless you want to place your bets as James Gunn being the one to make the MCU fall to its knees with two terrible underperforming films.
Of course, these sources could be wrong, and in fact Gunn could simply be in consideration for the job. If the Guardians Of The Galaxy post-production is still going, is he ready to just jump onto Ant-Man? The report also suggests that if they couldn't get James Gunn, they would get one of his siblings, Sean Gunn or Brian Gunn. Which, of course, is total bullshit: neither actor has directed, and while Brian previously wrote Journey 2: The Mysterious Island, Sean's career has consisted of bit acting roles in a number of films and television shows. That's not exactly the pedigree Marvel is pursuing for its big, risky summer movie.
James Gunn was the most outspoken member of the Marvel family when Edgar Wright left Ant-Man, writing a Facebook post about his regret over the situation without throwing either person under the bus. "Not everyone belongs in a relationship together" was his sentiment, and it seemed like the perfect way to soothe both Ant-Man and Wright devotees as well as those swearing at the altar of Marvel. Was it enough to get him this job? Guardians Of The Galaxy was sort of a goofy, broad palette, but now he'd be dealing with a movie set up by another filmmaker with two bonafide leading men in Paul Rudd and Michael Douglas. Big shoes to fill for a guy some say Marvel was absolutely crazy to hire in the first place.