Good news! Your friend, Professor Hypothetical-Question, has developed a time machine! Unfortunately, it’s pretty limited. It will allow you to do one thing and one thing only: Start an account for an MMO that is currently shut down in an official capacity back when the game initially launched. You will then be able to play through the game as a player in real time, one day at a time, until the game shuts down. So if you go with City of Heroes, you will have about nine years of play time before the game shuts down.
As mentioned, this is a time machine, which means that you will be playing the game from launch onward and dealing with all of the patches or instabilities along the way. And for some reason, yes, it only works on games that have shut down. There are a lot of those titles, though. So which game would you choose to play, knowing right from the outset what would be added and when and exactly how the story ends?