Plug-in Thermostat Switch With Wireless Sensor
This would be a device that you would plug into a wall and plug a heating appliance, such as a slow cooker or dehydrator, into the device. You would set the thermostat for the temperature you would like the food in the appliance to maintain, or possibly two temperatures: one for the appliance to turn on at when it falls below it and one for it to turn off at when it reaches it.
The sensor would be wireless, like some modern thermometers, and sealed such that it could be completely submerged in the food. Putting the sensor in the food is more accurate than putting it in the appliance, especially in a cold room.
Making it wireless and sealed would eliminate the problem of cords holding the lids and covers of appliances ajar. One could even screw the lid onto a jar with the sensor inside. This would be useful for anaerobic food ferments like sauerkraut. I would use it with a crockpot and a dehydrator that I bought at a thrift store.
Reward: A free one.