My Grandfather who is now gone invented and patented the first fish alarm. It was constructed in a fishing box basicly you casted your line like normal and then you sat you rod down and looped the string through a nifty little contraption ontop of the fishing box. When the string got a little jerk it fliped a switch and rang a doorbell which was inside the box.
When he passed away he left me and my brother with the only one in existance today. I think it is actually box number two.
But as far as ideas for you it seams like it would be fairly simple. Make a motor release button so you can release the motor gears from the spoil of wire and cast the line. When the line is setteled reingage the gears to the motor. Use some sort of switch to detect if the line has been tugged on a few time. If its been tuged on hard enough twice and real the motor will real in the line until the line gets tugged again.
The problem here is what if the line get stuck on a rock and the motor still wants to real in. Well its going to keep realing it but instead of the string comming in the rod is going to get pulled into the watter.
Plus why would you want to do this. The whole fun of fishing is feeling for the fish and then yanking really good to hook that little rascal.
Sorry but it think you need another million dollar idea!