Yeah, I suppose the Mexican wouldn't really fit.
Plot: The basic plot of the ride would be an earthquake cracks open the Money Bin sending Scrooge's cash down into the caverns. Guests must board mine cars to enter the caverns, find the cash, and escape the Terry Firmies who caused the quake.
For the comic that I'm basing the ride on go here:
http://disneycomics.free.fr/Ducks/Barks/1956/landbtg/Thumbnails.html
There's probably a Youtube video on the DuckTales version - just search for the episode called "Earthquack."
Ride system: The ride mechanics would probably be like a cross between Primeval Whirl and Big Thunder, with a least one upside down loop (just like in this page of the comic:
http://disneycomics.free.fr/Ducks/Barks/1956/landbtg/USAdventures_28-L11.html - The mine car scene from "Temple of Doom" also comes to mind when designing this.
Also, the Fire Mountain idea sounds pretty sweet - 10/10. I especially like how the restaurant would be situated with it. I'll give the Villains ride a 7/10 - neat idea but it loses points for carrying too many similarities to Fantasmic. I'll give the National Treasure ride a 6/10 - it loses points because I'd rather have an Indy ride. The Scottish Nessie ride could easily go into the old 20k Leagues area (that is if they dig it up and fill it up with water again) - 9/10.