I think you might be a bit confused. This isn't a sequel to District 9. Maybe the "follow-up" wording is confusing you. However, they just mean follow-up as in it's his next movie after District 9.

Blomkamp has said himself that this is completely different than District 9. He and Copley both have stated they want to do a "District 10", however they've said that's a couple years off. But, Elysium will be similar (despite being completely different), in that it will appeal to fans of District 9. It's a sci-fi movie with political commentary, aliens and violence/action. Here's how Blomkamp described the movie:

"Hopefully, this will be a bit unique, very much a reflection of me. It is absolutely another science fiction film, quite different from `District 9,’ but some of the blending of genres and the tone might be within the same realm.

It’s totally original, it’s my own story…It’s set on another planet, but it’s cool. It’s violent, very violent, and very unique hopefully.

It is science fiction and it has many sociopolitical ideas that interest me. Those ideas are wrapped up inside something that is like a Hollywood action film…The setting for the next film takes place 150 years from now."