October Daye/WMG

Evening was Toby's grand-aunt, or a relative of some kind.
Why else would the queen declare a changeling the Countess of Goldengreen, as seen in the preview for Late Eclipses?
 * Because Toby did a great service in during An Artificial Night?
 * And she wanted to

Quentin is a close relative of King Aethlin Sollys and Queen - the fae rulers of all of North and South America. Either a son or maybe a nephew.
Their knowe is in Canada (Toronto, I believe) and Quentin's accent (near Edmonton?) is mentioned a few times in the first two books. Granted Canada is Really Big, but they're the only fae in the series that are from Canada. The unusual aspects of Quentin's fostering situation are commented on several times - that it's blind fostering, that he's staying at Shadowed Hills until he's 21, etc. One reason for this could be that he's the relative of somebody Really Important and they don't want that affecting his training as a knight.
 * Possibly hinted at when.

One Salt Sea (Book 5) will feature the Luidaeg heavily.
This is just kind of out there, but she is the SEA witch and, well, duh. Something seemed to be odd about her at the end of Late Eclipses when Toby asked if she was okay.
 * Confirmed.

Before the end of the series, Toby will investigate the disappearance of Oberon and his wives/mates/baby mamas.
She is the hero who finds things. And they keep talking about it. And McGuire already set up this kind of long-foreshadowed ancient mystery bursting into contemporary life with Blind Michael. (Just wanted to get this down so when it happens I can totally point to it and say: Called it!)
 * And what she finds on this quest will destroy/drastically change Faerie, as described in.

Amandine is the sister who betrayed the Luidaeg's roane children.
The Luidaeg brought Toby along with her to deliver her warning to Elizabeth Ryan, head of the selkies, and pointed out who Toby had been lovers with and who her mother is right before she did it. That's probably significant somehow. "They would keep the magic alive until I could find a way to make things right." Possibly Toby is related to the reason why the Luidaeg gave them a year before she does...whatever it is. Maybe Toby's saving of sea fae helped this too? Also, Mary refers to Amandine as "the liar." Since she's one of the few remaining roane left, she'd have good reason to know that first hand.
 * And this will lead to some kind of showdown between Toby and the Luidaeg. They both keep saying the Luidaeg will kill Toby, but it never seriously looks like happening. And we know the Luidaeg is capable of loving and hating the same person - witness her feelings towards the Selkies. Also, Toby's been told to beware "the Lady of the Lake".

Toby is not an only child.
Oleander talks about the self-righteous disgust of Amandine's children, plus the Luidaeg says Toby's the last one who can be Amandine's heir, not the only one.
 * Oleander's words suggest she might've gone up against the others before Toby (and probably won)
 * Amandine's unorthodox approach to childrearing was because she's tried a bunch of nicer methods already and lost all her previous kids. Losing Toby to Simon and Oleander (like she lost all the others to them?) drove her crazy.
 * One of Toby's half-siblings might show up later