Hey! Some fan actually subtitled this one!
I am determined to plow through the last section of the Steamscapes: Japan outline tomorrow. I've outlined up to the arrival of Perry and skipped ahead to the aftermath of the Boshin War, but that section between the Black Ships and the establishment of the Republic of Ezo is where the alternate history happens and I'm not sure how much depth I need to go into.
In Steamscapes: North America, a lot of weird stuff happens but a lot of history runs parallel to the real world despite itself. The main difference seems to be a fragmenting of the United States of America into smaller republics. I'd originally thought that the Tokugawa Shogunate should win the Boshin War, but further study leads me to the conclusion that the shogunate was too deeply flawed to last. There's just no way a despised military dictatorship can win in the long run if it opposes the basic religious beliefs of the nation -- plus the Republic of Ezo is amazingly weird, so I think the setting would benefit from including it as an ongoing concern.
There's a fine balance to be maintained between "alternate" and "history."