Looking at the rules pdf, it seems like it's the base game, with a selection of characters from different expansions. The full character roster is in the rules and includes characters from expansions: Dodge City, Wild West Show, Gold Rush, Valley of Shadows, and one character unique to the 10th anniversary edition.
So to sum up, it's the base game with more characters.
Thanks. But, darn. That's what I was afraid of.