Sometime in the 80's I remember seeing variations for Risk in Games magazine. These have always been some of my favorite game variations, but I am going off memory here so stay with me...
1) Nuclear "accident" - Whenever the attacker roles 3 6s on an attack, draw a random Risk Card from the deck. All armies on that country are destroyed and the country is marked with a coin to indicate that it is irradiated. If unoccupied, the country is no longer counted when determining if a player owns the entire continent, however other players that border the country may move armies into the irradiated country as reinforcements. Half of all armies (rounded up) entering the irradiated country are immediately destroyed.
2) Nuclear Strike - On their turn, a player may play one of their Risk cards to destroy all of the armies on the country that is depicted on the card. The Risk card is discarded. The country becomes irradiated and is subject to the rules above. A player may not make any attacks or reinforce on the turn that they use the Nuclear Strike option and they do not get a Risk Card (so the player is effectively 2 cards behind the other players)