I find that the best mechanics are ones that radically restrict something that you wouldn't expect. Also, they should be very integrated into the theme. I'm making a co-op board game, and came up with a few ideas. In the game, that players are all astronauts on a huge spaceship that has broken down. They have to go outside the ship in zerograv, and try to repair malfunctions before the ship breaks down entirely. Here are my two ideas.
1.) Every player has a small number of battery packs. No one is allowed to speak or communicate in any way during the game apart from thumbs up or thumbs down signs. If a player wants to speak, he has to spend one battery pack, and start a sand timer. That's how long he has until he has to shut up. New battery packs will be available, but at a very slow rate, so players must conserve talking as much as possible. In a co-op game, this should stop one player from ruling all the others, and also introduce a lot of tension.
2.) Scattered around the board (the exterior of the derelict spacecraft) are oxygen valves. During a turn, all players go take turns performing one action with their characters (moving, examining a malfunction, drawing a repair card, picking up a battery pack, etc). They are only allowed to BREATHE when they are at an oxygen valve. They must literally hold their breaths during a round, waiting for each other player to take their one action before doing theirs, making their way towards strategic points or air valves. If they breathe, then they are dead and eliminated from the game. Each round lasts the length of a sand timer, when it runs out, the only moves that a player make make are movements back towards an air valve. My thoughts: this could be the most intense and thematic game mechanic ever, or it could be really annoying.
Overall these mechanics should make the game both truly silent (where another person's voice is a welcome relief, and no one talks unless it is REALLY important) and breathlessly intense, as people feel like they are fighting for survival. (Dead Space would be the best name ever, but that video game took it.)
Wow, it looks like multiple copies of this got posted. And BGDF won't let me delete the extra copies. I'll try again later.