I just finished Law of Nines last night. It was better than I was expecting given teh way it sounded from the book description.
There is room for a sequel, but I think that any sequel that gets made would have to involve the SoT world more than LoN did.
The trick to that is... How do you set that up if you're still telling the plot from Alexander's point of view? If no one from his world can go through the gateway safely because they lack the spark of the gift required. Though an easy way to set that up would be to say that his connection to the other world is evidence of a spark of the gift, which would make sense, because Richard claimed that the new world would have no magic, and thus, no connection to other worlds... But if there are no connections... then how did Alexander see the woods Jax liked to hang in?