One of the suggestions submitted by Miguel Balauag from Stanford:

This variant is only playable when dead players reveal their cards. The moderator selects the civilian cards in pairs (i.e. two 4s, two 5s, although I recommend doing colored pairs, such as the two black 5s, etc. to allow for more pairs). Players are grouped into "families" this way, although they do not know who their "relative" is. As normal, the Mafia kill a player at night, and the dead player's card is revealed at daybreak. During the next nightfall, the moderator wakes up the "relative" (the player knows who he/she is because he/she saw the revealed card). The "relative" can choose to take revenge and kill a player. This allows for people to eliminate a player they suspect of being Mafia, without having to sway a majority vote during daybreak. Of course, it's not always wise to blindly take revenge without a good suspicion.

This variant actually also can lead to a draw in a specific case which Mafia would normally win. Suppose at nightfall, there are three remaining players: 1 Mafia, and 2 Civilians who are related. At night, the Mafia kills one, and during daybreak, the lone Civilian can not get the Mafia lynched. During the next nightfall, the Mafia kills the Civilian, but the Civilian also takes revenge and kills the Mafia, ending the game with no living players. I imagine this would be a rare case, but it would need some definite ruling as to who wins, or if the game simply ends in a draw.

When we tested this variant on 2003-01-09 (see notes), we found that there could be an escalating number of Vigilantes when both Mafia and Vigilante kill different "civilians" at the same time... And also, we played a game where all civilians were paired, and thus each was uniquely identifiable, resulting in a situation where Mafia HAD to loose! Or did they?