I would replace the existing iMac keyboard and Magic Mouse with the new rechargeable Magic keyboard and Magic Trackpad. iMac 21.5" is my primary computer and I use it a lot, thus requiring change in batteries of the keyboard and mouse every 15-20 days. That is really painful. Rechargeable keyboard and trackpad would do wonders for me as buying the Magic Keyboard and Magic Trackpad here in India would mean spending ₹15000 over them, while a new HP laptop is available at ₹22000. Damn expensive here. Very much in need of the rechargeable keyboard and trackpad but holding myself due to the price...
Additionally the force touch on the trackpad would help me accomplish so many tasks without having to go through the long processes...