You have already explored speculative fiction a bit… Leviathan is part of what is referred to as Steam Punk. Have you encountered this before? What makes this different from other fiction you have read? Who is going to get excited about this literature (there are diehard fans) and how might you build on that excitement in the classroom?
Here is a link to Westerfield’s blog.
http://scottwesterfeld.com/blog/
Trailers for his novels are also on this site
Childrenslitblog's Weblog 
I haven’t read anything considered “steampunk” before and it is a really interesting idea. It takes a very unconventional look at the past world. I was fascinated by the imaginative writing and I liked looking at the illustrations and trying to figure out exactly what these things looked like. By combining the past with the future, surreal world, I could see it helping students to understand history, while presenting the material to them in an unconventional way—a more creative world than the real one.
Kids who like science fiction, video games, cool, gross things—I could see it being a real draw for young boys. I enjoyed it as a 21 year old female as well but it depends on what draws students into reading. There are machines and a boy who pretends she’s a girl and females are looked down on so I could see it primarily targeting boys or tomboys.
The excitement is easy to build on in the classroom because all you have to do is start describing it and the expanse of the whole machine or the creature and I can see students’ eyes widening, trying to wrap their minds around these creatures. Art and illustration in the classroom would also be useful because there are so many images. Discussions about what could possibly be in the eggs at the end, or speculation on the characters of the woman or what it would be like to run one of those machines or ride in one of the creatures. 3D models could be made, history lessons could help improve understanding…etc. Lots of possibilities.
Steampunk gets a little wilder than Alternate Histories… but I think you’re right that it excites the historical imagination in new ways. The illustrations are cool in this case but they also help readers imagine what these creatures and machines look like.