^ It's tough for Luke to acquire more saber feats when every other Force user he could've sparred with is so much weaker than him (and Sidious). He was already beginning to acquire some haxxed Force feats like destroying Vader's fortress and cloaking a capital ship, but I'm not sure if they're quite clearly beyond Palpatine yet.
He did beat Palpatine in DE with the help of "unlocked resources" from Leia's battle meditation. I would wager than 15 years or so would have unlocked enough of that potential for him to defeat RotS Sidious.