Convert a surround-sound receiver into a multi-channel amplifier by removing everything except the power amplifier section and power supply. Use the outputs from this controller board to drive the power supply switching relay and any speaker protection relays. Use the inputs on this controller to interface with a thermal sensor (like the LM61CIZ used in an Onkyo TX-SR705) and a trigger input to turn on the amplifier. The ATMEGA can be programmed to drive LEDs indicating the amplifier's state, and apply delays to protect the system during startup.
Build your own power amplifier that includes thermal management, trigger inputs/outputs, and delayed relay firing at startup for protection circuits.
Overall, this PCB is quite similar to an Arduino, but with a little extra circuitry, and not as many general I/O pins exposed as such.