For my the best iPad that could exist will be an iPad with a Full OS X and a "layer" like FrontRow or Dashboard that will put the OSX in an iPhone like version.
IE the iPad will work mainly as an iPhoneOS device when used standalone, and when I plug it to a special dock that give it more peripherals (like touchpad, keyboard, DVD driver, bigger battery, and lot of connectivity (USB/FW/SDcard etc..)) it work like a classical OS X, but with multitouch screen in bonus!
That's my ideal iPad
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