This is going to be my master post of Infernal Devices theories I see floating around. I will put ALL THE THEORIES here, even if they are completely horrible or implausible or make me want to bang my head against things. If you don't see my commentary in small letters after the theory, you can assume that I have no logical reason to object to it.
Also, I will organize them by character.
* indicates a theory I formulated myself (EDIT: crossed out= a theory that was wrong)
- She is part-Shadowhunter (in other words, she has some angel blood)*
- Follow-up: her mother was a Shadowhunter, who was switched at birth with a mundane/faerie-- Adele Starkweather. This is why Adele's skin burned when she got her first rune; she's not a Shadowhunter. (I love this theory and think it's probably correct. Also it makes Tessa a Starkweather WHAAAAT)
Her "second ability" has something to do with visions of the future or visions of present things that she can't see in person*
- it is also somehow related to her clockwork angel*
- OR her second ability is the power to summon angels (which seems probable based on the Clockwork Princess jacket copy)
He is Brother Zachariah (possible, but I don't think so, because the clues so obviously point to him. The clues are intentionally misleading; Cassie said so herself.) He becomes a Silent Brother as payment to them for curing Jem (unlikely-- if the Silent Brothers knew of a way to cure Jem, they would have done it already, and I'm fairly certain they don't require payment. Also, Jem would never let him do it.)
- He will end up with Tessa but will die of old age or whatever before her because she is immortal
He will die/sacrifice himself in Clockwork Princess (Nope. Nope nope nope. Just... nope.) He will not end up with Tessa and instead will marry someone else in an arranged marriage (right, guys. Because Will is so keen on having other people make such decisions for him. Also, who would make this decision? His parents haven't been a part of his life since he was 12. Charlotte would probably be both morally opposed given her own arranged marriage, and would see no use in even trying to compel Will to do something like this. He sure as heck wouldn't listen to the Clave, and even they probably realize it would be hopeless [I mean, remember the deleted scene where they yelled at him because of his clothes and he showed up naked the next time?] Plus, he says it himself in CP2: it's Tessa for him, or no one. I just... this is one of those theories that makes me wonder if people who come up with theories actually read the books.)
- He is Brother Zachariah (possible, but again, I don't think so. The clues also point to him, but it's not just that. I don't think Jem would become a Silent Brother just so he wouldn't have to die. He's been living with his imminent death for long enough now that he's made peace with it. Of course, he's not okay with it, but he believes in reincarnation. He doesn't see death as death. He doesn't think he needs immortality to stay in this world forever.)
He is the Magister (again... there is not really any textual evidence to support this, other than the fact that both the Magister and Jem want to marry Tessa. Because apparently only one man can want to marry her, and it can't be for any reason like he actually loves her or anything.) Jem dies in Clockwork Princess (the way one would expect Jem to die, i.e. not of old age) He ends up with Cecily (possible, but I don't really think Jem's going to look at any other females while engaged to Tessa. He's not that kind of dude. Though I'm sure Will would be totally for it.)
She is a spy for Mortmain (seems totally possible, except NO because WILL.) She is Brother Zachariah. (INTERESTING but as far as I know Silent Brothers are dudes?)
- She marries Gabriel and this is why the Lightwoods end up with Herondale appearances (no textual evidence yet but I don't see why this can't be true, other than the fact that there are several generations between TID and TMI, during which looks can change a lot.)