Agreed that according to the current Lucas system, Qui-Gon is the only force-ghost other than Ben, Yoda & Anakin who could appear at the end of RotJ. However, just because he can, doesn't mean he should. That scene is about Luke seeing that his father has indeed redeemed himself. Luke knows Ben and Yoda, but doesn't know Qui-Gon, so there's no reason for Qui-Gon to appear to Luke. Qui-Gon also could appear on Dagobah in ESB or RotJ but doesn't - so why here?