Mortal Instruments/WMG

The Clave would want to sacrifice Jace in order to destroy Jonathan.

• Yeah... but it didn't happen in the end.

Alec or Magnus will die.

• But what about all the Unfortunate Implications?
• It actually is highly likely seeing as, in response to a question about what happens when somebody's parabatai dies, Cassandra Clare responded with a "You'll see."

Clary will use the bell to get a favor from the Seelie Queen, i.e., getting Jonathan out of Jace.

• Confirmed somewhat.

Maureen will return to be a major character in the climax.

Somehow, I don't think Cassie Clare would introduce a character give her a blog without her being influential later.

Magnus will cast off his immortality in hopes of spending the remainder of his life with Alec.

• Magnus has already said that he wouldn't turn Alec immortal, but the book of white may have the reverse to that spell.
• This is the most likely scenario for their relationship (especially given how things go in City of Lost Souls).

Alec has been convinced by Camille to seek out non-vampire ways of becoming immortal, and will succeed

• Combine with WWG above for the most angst possible.
• Alec does seek out Camille.... but becoming immortal for him is impossible so he decides to consider turning Magnus mortal.

Clary will really develop feelings for Sebastian

• ...thanks to the Infernal Cup(since she did taste a "bit of blood" before breaking free).