I just thought about a couple of things:

(1) maybe the arm stump could be modified further to show a metal "bone" coming out of the stump, ie. a mix of the Terminator endo-skeleton mixed with some other components (as opposed to just generic wiring at the end)

(2) I always had a problem with Vader taking a knee(l) when Luke is whacking at him - it always felt like Vader wasn't beaten down by Luke so much as David Prowse was told to just get down and let Mark/Luke whack away. Maybe some editing magic could be used to take that brief scene out (Vader fights back, cut to Luke whacking away/ cut to Vader already down with Luke pummeling him/ then cuts arm off)