The 39 Clues/WMG

Everyone dies.
The series appears to be on its way to this, and we've already seen several examples.
 * What!!! But it's a book aimed at kids. I don't think they can do that :O
 * Jossed.

Arthur Trent is Vesper 2
As stated in book 2 of Cahills vs. Vespers, he is  and Vesper 2 makes a statement about being able to turn Dan and the.

Broderick T. Wizard is/was a Vesper.
It's possible he met Cora Wizard,fell in love with her and married into the Cahill family, like Arthur and Hope. Let's not forget the clues Grace left on his cards...

Isabel Kabra is a Vesper.
Her maiden name is Vesper-Hollingsworth after all. But it makes the suggestion that the Vespers make her look like Mother Theresa an odd thing to say if she's really a Vesper.
 * Unless the Madrigals simply aren't aware that she is one...
 * Didn't that one get confirmed? Unless I haven't read the books attentively enough... But it's on the wiki...
 * Well, it hasn't actually been confirmed, but it's been heavily implied that she's a Vesper, or at least involved with them.

In Book #3 of "Cahills Vs Vespers", will be a secret code that once decoded will read: Madeline finish it.

 * In the Black Circle, they were secret code that revealed two secret message: "Olivia had another" and "Remember Madeline.". This was revealed to Madeline Cahill, the fifth child of Gideon and Olivia Cahill, and founder of the Madrigals. Since half of 10 is 5 and half is 6 is 3. Book #3 should logical where this code will be hidden. It is the Gideon Cahill's second version of Master Serum.

Evan Tolliver will turn out to be a Vesper agent.

 * Let's face it: Amy's been screwed over by love before, we've never heard of Evan until Cahills vs. Vespers, and the theme of the series is "trust no one." There's a pretty good chance.
 * By the time the first two books in Cahills vs. Vespers are finished, it's been made pretty clear that Evan truly had no clue that Cahills were a special family, so he can't be a Vesper agent. However, that doesn't mean he can't be turned to the dark side...

Grace Cahill was a Timelady.

 * First of all, it seems mysterious how nearly every major historical figure for the last bunch of centuries had helped set up a competition, when a single mishap could have ruined it all. Second, the Cahills seem way too spread out, even with the children moving around. Obviously, Grace's secret library would have been her Tardis.

Arthur Trent and Damien Vesper are the same person.

 * The Black Book of Buried Secrets confirmed that the Trent family, right? What could be closer than   (Before anyone asks- yes, Arthur  )
 * Although
 * The three of them are ALL the same person. And that person is Vesper 1.

At the end of Cahills vs. Vespers, Amy will handle her tangled relationships by...

 * ...telling all three prospective guys to leave her alone to cool down for at least a few months. Not only is this arguably the most logical choice, it won't severely upset many fans, while at the same time will avoid promoting breaking up with a nice person in order to get with your Tall Dark and Handsome stalker (said message would definitely not be met by approval from parents and older readers).