Batman Arkham City

[Frankie_Says_Relax]

You may need to do another playthrough to finish off all the side things, depending on what you did during your run. A lot of quests become inaccessible after the game has been beaten.

I've seen people say that online but it's not true.

All of the side missions can be brought to 100% completion after the main story mode is finished. Once you wrap up your business with Joker & company and the credits roll you're free to take care of anything/everything that Arkham City had going on during the main game.

The ONLY limitation similar to what you're describing is that during the final chapter of the main story mode (when the GCPD is in the city) the side missions lock down.