Just in case no one has properly suggested this kind of thing... the man with his hand on the kill switch.
Role name: Bomber/Demolitionist
Description: You are an unhinged explosives expert and delight in secretly blowing up all who oppose the Mafia.
Alignment: Mafia Support (killing)
Abilities: Possible night actions...
Plant explosive -- visit a target to hide explosives in their house. You have one bomb.
Detonate explosive -- by targeting someone you have visited before, you can detonate the bomb at their home. Anyone visiting the target that night (except you) will be killed by this ability. The target is considered to visit himself if they do not target anyone that night. If no one visits the target that night, they will get a message in the morning saying that their house was blown up while they were out.
Defuse -- by targeting someone you have visited before, you can remove the bomb from their house if you change your mind or if they have died. You regain the use of a bomb if you do this.
Win Condition: See those who oppose the mafia dead.
Host Options:
1) target knows he has been visited
2) bomb can be defused and recovered
3) excluded from randoms
4) can target self
5) becomes mafioso if alone
Pros: Adds a fun mafia killing role that brings just the right amount of danger to the town. This role would reward careful play, and change the stakes of any game at least a little. I thought about more than one bombing, but the bomber/demolitionist (I couldn't decide which name was better) can potentially kill more than one townie with his ability, unlike the kidnapper or disguiser. Since those roles have greater implications beyond their killing capability, though, and since the bomber not only has to take two turns to get his kill(s) off (which aren't guaranteed) but also has no power after his kill, I figured the potential kills were fair. The kidnapper can continue to pretend to be a jailor, and the disguiser pretends to be whomever he replaced, after all. The bomber cannot.
Cons: Very few. The role could be wasted in the wrong hands, or with bad luck, or if the mafia doesn't pay attention/communicate they could end up blowing up an ally. The bomber/demo could also get TOO lucky and end up killing too many townies with his one-shot. Bomber may also get antsy once his bomb is used. These are unlikely, though, since the use of the bomb requires planning. Other classes have a greater effect on the game if used incorrectly, by far. Just look at fail vigi, marshall, or jailor...
Overall effect on the metagame if implemented: Fun, fun, and fun. I think the strength of this role is the fun it would bring to the game. Once there's a suspicion that the bomber is in play, the feeling of fear is similar to that presented by the potential mass murderer or kidnapper. Everyone has to be a little more cautious. However, unlike the mass murderer, the bomber cannot waste his spree by using it as early and as often as possible. This role would create a number of interesting scenarios, though. In cases where the target knows he is wired with explosives, he has some interesting decisions to make as far as how he handles his next and potentially last day. Doctors and bodyguards would essentially have to 'jump on the grenade' if the mayor was wired, for instance. By healing the target on the night of detonation, the protectors would die but the target would live. So, docs should be able to prevent the bomb from being placed at all, kind of like preventing an arsonist douse.
Bomber/demo would also appear as vet/mm/demo to investigators, as 'owning lots of weapons.' This is a nice place for bomber considering that it, like the other two roles, can kill multiple people in one night, and, if I'm not mistaken, there's no mafia role that appears as vet/mm.
Additional comments: That's about it. I considered making this role more of a "car-bomber" that punishes townies that leave their house, e.g.: you plant the bomb and if they leave their house the following night they get blown up. But I decided on this one finally. A completely different class could be written around that I suppose. Also, there's room for a "suicide bomb" style ability, I think, I'm just not sure where or how. Could the bomber walk into a building with a bomb strapped to himself after he's used his first bomb? Should he be able to blow up anyone who visits him one night (and perhaps himself with them?) Finally, any more than one bomb would be silly, right? Please give feedback! Also, consider adding this -- insert it where you would use kidnapper or disguiser!