Spark plug hole cleaning tool.
I don't know if this exists already, but it should. It is important to keep spark plug holes nice and clean for several reasons. The trouble is, they are hard to clean with the plugs in place, and with the plugs out, the easiest place for the crud to go is into the cylinder, which could be harmful.
So this tool would basically screw a few turns into the threads of the hole, and then allow a wire brush to be lowered and rotated to clean the metal around the hole. It might also have a compressed air inlet, allowing you to blow the crud out and away after brushing. The outlet for this air and crud could go through a small mesh or bag to avoid having it go all over the place. It would be available with a few different brush sizes for different applications; the fancy version would incude all of these and they would be easily interchangeable.
The part that threads into the hole would be designed to be very easy to start, to avoid damaging the threads, and either would be designed to also clean the threads, or there would be in interchangeable tip, to clean or not clean them. With the cleaning tip in place, the compressed air would be introduced into the cylinder through a center hole and blow any shavings or crud back through grooves cut in the cleaning head.