WARNING MAY CONTAIN SPOILERS
okay so this is my prediction of the infernal devices and mortal instruments! (it might be some what flawed so yea..)
So Jem and Tessa are all in love and happy, so William LEAVES to become a Silent Brother (Brother Zachariah to be exact! [I have heard guesses that Jem is BZ, but I don’t think so.]) So while Will is becoming all creepy, Jem and Tessa find out that her father is the same demon that murdered the Carstairs family. Jem just cannot handle this and leaves Tessa, who is in so much pain leaves the Institute and moves on and is not to be seen for a really long time. I know what you’re thinking “If Will is gone what about Jace?” Well he still has a sister right? Well I think she is going to end up with Gabriel Lightwood (who ends up dying before the baby is gone or he leaves him) so the baby gets the Herondale name. So the Lightwood name is continued by Gideon Lightwood and Sophie (maybe?). And the Carstairs name is continued by Jem (or maybe this mysterious Emma who might be a sister?)
Well Father Time passes some gas and we end up at the Hall in Alicante where the people left over from the battle are a-talkin and enter Mr. Wayland (Even though he is dead!) I believe that Tessa posed as him and told everyone this story for Valentine! (this part might be totally wrong! If so please ignore and continue)
Okay so as those of us who are up to date know, Magnus and Alec sadly broke up. I am guessing Magnus has also sworn off Shadow Hunters as well, but he isn’t mean! He called in Tessa to HELP! And lets sat that Brother Zachariah is magically there! They meet and BADABOOM! Rekindled love! He leaves the brotherhood and Tessa fixes his face with magic! For the rest I don’t care!
THIS IS MY GUESS! IF I AM RIGHT YOU OWE ME $20 BUCKS!
just kidding. I was really just bored! :)