How about new players with very long battery life?

Some find having battery life of 40+ hours(and especially 60-80 hours) to be very useful. How about a player designed with a battery slot(similar to the idea of slotradio where you can choose what capacity you want, but for battery power instead of memory storage). There could be batteries in different thicknesses that snap on to the back of the player for different power capacities. Motorola did this to some degree with the Razr cellphone, having an optional jumbo battery that replaces the standard one( It also comes with a special larger battery cover to accomodate the larger battery’s extra thickness). Of course such a plan also means that the battery needs to be easily swappable, and there should be a charger to charge spare batteries outside the player.