It would have been nice to see a live action Darth Vader that could also move like Anakin could. I realize that in the 70s/80s it was slightly difficult to get a man in a giant suit move with any semblance of agility, but it would have been epic to see Darth Vader do any actual fighting.
I just can't imagine that looking good at all, even with today's technology. A lot of what makes Vader seem so powerful and menacing is the slow and deliberate way in which he moves.
If you have him suddenly flipping through the air and doing acrobatic moves at lightning speed, that just wouldn't come through nearly as strong, I don't think.