I believe that the next master sword should be a samurai sword. And, since it's the samurai sword and you can't use a shield while properly using it, it would allow for a new and awesome combat style. Or, just make one of your customizable weapon schemes a samurai style, along with the sword and shield, and spear style.

Like people have said, there should be more magic in the game. I really liked that one about the earth, water, fire and air. That was cool.

Maybe they could replace that useless roll attack with a grabbing ability. Grabbing would be awesome. You could get strength upgrades or gauntlets so that you can grab'n throw bigger/tougher enemies. Through enemies at each other for some awesome combos. Mopping them up faster.