I know there is already a "brake" key, but having reverse thrusters on the down arrow would be much more natural and useful and easier to use. They wouldn't be that hard to implement, either--especially relative to alot of other suggestions on here. For speed balance, reverse thruster power could be set to 70% or so of main thruster power.
I've already tried to add reverse thrusters using the mod system. My plan was to make a "RetroRocket" mod, basically a "missle launcher" type device that when fired would provide negative thrust. Unfortunately, it seems that the game treats an engine with negative thrust as if it were zero thrust, so this can't be done with the current version of the game engine.
