The biggest addition to MyGM is a narrative that is similar to the stories that played out in the MyPlayer/MyCareer modes of past games. The MyGM narrative only lasts for one year but is certainly interesting and can increase or decrease your enjoyment of the mode depending on how you feel about it. The gameplay additions were the best part in my opinion, as NBA 2K18 got closer to being a perfect simulation of being an actual general manager.
The story opens with your avatar blowing out his entire knee during the 2011 Western Conference finals towards the end of a game.