Well since there's some other interest, I thought I might posit the idea of of a series set another 50-70 years after the first one, where Ancelstierre is an equivalent to our modern world, but the Old Kingdom hasn't really changed much(besides civilization expanding further since the restoration). Internet has made the existence of magic to the North much more widely known, and there are groups from Ancelstierre who actively seek it out now, which cause problems when they think the Old Kingdom is going to be all magic and wonderment, and don't understand the horrors that live there.
There's also room for the Abhorsen family to expand some. If we assume Lirael has children at some point, then both her and the Royal family have their blood and could produce possible Abhorsens In Waiting.
Half a century out there's a lot of room for our own plot ideas.
Fading Memory,
Iracebeth,
Not a Sayan,
Neziul,
abc,
Fetzen,
Xaltwind,
Veilsight,
FlappyTheSpybot