Couldn't you make a wireless mouse that charges though the mouse pad such as wireless charging phones, surely that way you would not have to worry about ever running out of charge or remembering to charge it
20 hours of gaming on 2 AA batteries? My wife's mouse runs for the better part of a year (~30 hours a week of use) on a single AA battery. Granted it is not a fancy laser mouse, but something tells me that there is a lot of overhead on this poor little mouse that could be cut.
The ON/OFF switch is the weakest link in that design since the button will eventually give way and cause all sorts of funky behavior. I own a MS X8 mouse and the repeated use of the button on the bottom to shut down the mouse as I used to be away for long periods from my machine has left it crippled. Now I have to deal with my mouse shutting down randomly cos the mouse pad accidentally brushed against the button and knocked it off.