Matt Damon. Pretty cool concept over all. Execution will make it different ir not. Overpopulation was a big difference between yours and Elysium.
A generation spacecraft made up of a number of o'neil habitat cylinders (Rama-like if you prefer) - in which the inhabitant's have forgotten that they're on a spaceccraft and treat it as the world having regressed to a ston eage tribal subsistance level (due to an accident in the remote past).
Story centers around one of the characters gradual awakening to the real nature of the world and the the peril that they are all in.
Oh yes - there are also intelligent rats.