I was thinking of a similar idea, but including both North and South America as well as western Europe and a little bit of Asia. (Possibly Africa, but I'll probably make another Race for Africa scenario Risk.)
Adding an economy system was too difficult, so I thought that the Economy should also be the bonus when rolling.
You could choose between two groups: European or Natives.
Europeans got bonuses for moving ships from their colonies to their cities. Ship generations, naval combat and blockades are important. The farther
Natives got bonuses by taking colonies. They got x5 of whatever the Europeans get by simply moving their ships back and forth. Huge bonus for taking European cities.
To take a city or colonies, you simply must surround it with your units. Making a colony expends a ship.
Europeans get 2x ship production and 0.5x colonization, and Natives get 0.5x ship production and 2.0x land wars on their home turf.
First European to get to Asia gets a big, one-time bonus.
Europeans are not allowed to war each other on their home continent until a set amount of turns in.
Just some ideas, may or may not turn into reality.