Maybe this kind of role was already proposed, I dunno, haven't read everything in this section of the forum. Anyway.
The Samaritan, is a Town Role, obviously, who would fall under Town Protective. He would be kind of a Reverse Disguiser.
Ability : Target a player at night. If the target is attacked, the Samaritan is killed instead, and switches identities with the target (like the Disguiser does). The Samaritan cannot be healed when his ability is activated, obviously.
Options : Target saved is invulnerable the next night - (ON/OFF)
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So the goal of this player, like the Bodyguard, is to die. Unlike the Bodyguard, he can't kill the attacker, but he protects his target for more than one night - first when he switches IDs, which can help a Sheriff or an Invest to get a few quiet nights after they reveal ; then if the 1 night invulnerability is activated (which can allow a double-check by a vigi, for example).
I think that this may be interesting, since it would allow a lot of mind games - some Maf players could claim that they were protected, or a Jester use it to get lynched, or a Citizen could use it to make Mafia target them instead of an important role. I always thought that the Disguiser had no equivalent in town, even if this is one of the most challenging roles (you have to assume someone else identity - easy in pub, but with good players, it's quite hard not to be spotted, especially if your target was vocal).
Plus, unlike the Bodyguard, the Samaritan should not always guard someone - you don't want to save a Maf from a SK and give it an alibi, or to waste your ability on a neutral or a Citizen.