How would the handheld play Wii games? Would you plug a nunchuck in and swing the console like the Wiimote? Would you sync a Wiimote and nunchuck to it and set the console down like a mini-TV?
People kept flinging Wiimotes....... wouldn't the console get flung even easier if it was the Wiimote?
As for a handheld that could emulate say up to the PS2, I'm not sure. Given how light the later PS2 models were, is it needed? The slim PS2 is lighter than some tablets. Definitely lighter than a few of the books I sometimes have in my backpack....even a couple of games, a controller, and a set of cables is lighter.
Probably no more than 300, and at that price point, it likely won't happen for quite a long time.....probably longer than it'll take Sony to rework a PS2 game or two for the Vita, or whatever they put out after it.