Pegg told Empire magazine about the movie, which starts shooting next April with Superbad's Greg Mottola directing:

I’m very, very excited about it. I think it’s a bigger film than probably Shaun of the Dead and Hot Fuzz put together... We’re figuring [the logistics of the alien] out at the moment and having real fun, working closely with Double Negative who did Shaun and Hot Fuzz and Cloverfield and Hellboy II. We’re looking to really work to find the best way to do it and to create something which is utterly believable and sympathetic and has gravity and presence and is somebody that you totally buy. He’s the eponymous hero, so he has to be amazing.

As great as this movie will undoubtedly be, it's not the much-talked about third movie in the Shaun/Fuzz trilogy with co-writer and director Edgar Wright. That movie, currently titled World's End, is still in pre-production, while Wright works on his adaptation of cult comic Scott Pilgrim.

Exclusive: Simon Pegg Talks Paul [Empire]

Following three Biblically-titled series (Armageddon, Revelations and Number Of The Beast), the Earth has been knocked off its axis, London has been destroyed by a crashed spaceship, and entire cities have been flooded and left unhabitible, all thanks to a battle between superhumans that didn't really go as well as could be expected. Wildstorm editor Ben Abernathy explained the thinking behind permanently changing the status quo of this particular superhero universe:

Looking at the landscape of the industry, we realized we needed to move our universe in a different direction, something that the “Big Two” couldn’t, or wouldn’t, do for a long period of time. And we decided that direction should be toward a sci-fi/horror direction of a post-apocalyptic setting (to a degree, an almost logical extension to where the [Wildstorm Universe] has been headed for years). There have been “visions” of a devastated, bleak future in other mainstream super-hero books, but nothing with the lasting impact or direction that the World’s End books will be tackling.

The World's End setting definitely isn't a cosmetic change for the line; former fan-favorite books have been relaunched with significant changes, including depowering (and deforming) half of The Authority before stranding them in an electricity-less radioactive former London and placing the Wildcats in charge of the sole refugees of a dying New York City.
This isn't the first time that the Wildstorm line has gone all out for change; in 2003, their Coup D'Tat series had the Authority depose the US Government and set themselves up as dictators of the United States, before they realized that they couldn't be bothered with all the paperwork. But with fans still hurting about 2006's failed Worldstorm relaunch (which died when Grant Morrison's core two series Wildcats and The Authority disappeared after one and two issues, respectively), will even the end of the world get them interested in checking out the series again?

Possibly not - comic fans are a notoriously fickle bunch, and I say that as one of the more fickle - but they should; although the lead-ins vary wildly in quality (Armageddon is fairly avoidable, and Revelations could've been half the length; Number Of The Beast, though again overlong, is very enjoyable, however), the World's End relaunch works.With this shift, the books have not only become far less of the generic superhero titles that they used to be, they also move outside of the superhero genre in general, having more in common with movies like I Am Legend and 28 Days Later, and it finally gives them an identity that isn't just "Superheroes who are a little bit more violent than the more famous ones," and watching characters who are more than slightly responsible for the end of the world as we know it having to be responsible for trying to rebuild society - while dealing with their guilt is, if nothing else, different from the traditional superhero angst. I'm not sure about the longterm prospects of the project - and hope that there's no deus ex machina planned, even though they've already partially set one up at the end of Number of The Beast - but for now, World's End is doing something that very few superhero comics can do: Showing what happens when everything goes wrong and no-one can save the day anymore.

Wildstorm: World's End [DC Comics]