Change dice roll algorithm to one based on the site "random.org". They use atmospheric fluctuations I believe to simulate randomness. (They even have a dice roll generator). The one currently in use by Risk has deficiencies which are painfully manifest when playing (eg. every once in awhile all players will have a string of extremely bad "luck" with their dice rolls)
Simply add the colors black, white and grey to pieces for variety.
Have you ever considered time or turn limits for an option for game settings. Sometimes no one will attack in which circumstance a game will drag on indefinitely. With this option, victory would be granted to one with the most units at the end of prescribed turn count e.g. 30 turns.
Enforced alliance treaties
You can agree to an alliance for say 3, 6, or 9 turns and you will not be able to attack an ally's territories for as long as it's in force unless with express permission for intervening territories (so a little checkbox comes up on ally's screen with "yes" or "no")