It has been a mystery ever since Cassandra Clare confirmed someone from the books will have to leave, for good. Death is inevitable in the Shadow World, and yet, it still gets her fans on edge. Nobody wants to see their favorite character die after all.
Now, most theories involve the main characters, since Cassandra also confirmed she will most likely kill up to four main characters in the series. Since Simon Lewis has recently lost his Mark of Cain, he has become the top guess. This can be backed up by several hints and threats throughout the series. It is said that by having this Mark he would be an outcast, and have many great enemies. Raphael Santiago, for example, has even swore to kill Simon as soon as the Mark was banished from his forehead. Though I personally am scared for his safety, I also doubt that he'll be the one killed. For one, he is Clare's favorite character and he would be such an obvious choice since his Mark is gone.
Besides Simon, the second most popular choice is Magnus Bane. Everyone loves the Warlock but Cassandra is known for killing off loved characters several times throughout both of her book series. (The Mortal Instruments, The Infernal Devices) Magnus is over 800 years old, so his story would have to end sometime. It would only be fair that if he weren't gifted mortality with Alec, of Alec turned immortal, the two would die together. Because of their break-up, however, this caused many theories about how he will die. The fans have raised chaos and demanded answers, which Cassandra slyly evades.