The Parasite/Ghost would be the same role but whichever name you feel would better fit mafia.
When you spawn as the parasite/ghost you are given one of the town roles at random, it will be a role of one of the people in the game. This initial "body" is weak, like if it says you are a doctor you cant heal any one with that body, or if it says your a vigilante you cant kill any one. What you do at night is find another host, you chose your target at night like most roles, visit them, and then infect them. Also after you leave your initial body have it be a shell, it cant do any thing and it just stands around for a few days then drops dead. If some one else visits the target, you have a 50% chance of going in to the other. The next day, you now control that person, their vote, their words, their actions at night, everything. You also see their role, and if you are mafia/mason you can talk at night. However, that should be an option to toggle during the day, so you can be in your target with out them knowing if you wanted. The parasite/ghost wins by being one of the last people alive, gaining more points for the fewer people alive. Like if you were possessing a lone SK you get more points than if you won with 3 mafia
Now, the parasite/ghost has some drawbacks. You can only stay in a body for two nights, you HAVE to leave by the third or you die/disappear, and then the next day it says "the parasite/ghost is dead/gone" If an investigator investigates you while possessed/infected it tell him the regular info then adds "your target's behavior is very odd...". If a doctor heals the person your in you die, and the next day it will say "The Parasite/ghost is dead/gone". If you are role blocked by a escort/consort you are forced to take them over. Also, if the body your in dies for what ever reason, its over for you.
I think if this role were to be implemented it would be a Special role, separate from neutrals, towns and mafia and only 1 special role can be in a game at a time.