First, I think that all 6-piece sets should have 7 pieces, but require only 5 of them for the full bonuses of the set. This would increase build diversity by allowing more variety in supporting items than what we have now, which is close to nil.
Second, I think the six-piece, one-skill set is the wrong direction for Blizzard to be going with sets, and disastrously so. It kills any build diversity in the game, and it leaves players out in the cold who might enjoy playing offbeat builds with “unpopular” skills; those skills having no “set” that improves their performance.
Don’t get me wrong, it’s fine to have a set like that, as long as for every one of those, there is another set that any class can wear, which focuses on performance bonuses rather than a single skill bonus.
This is not to say that a “performance bonus” cannot include skill bonuses, but that bonus must affect all skills for all classes, or all skills for a specific class, or a specific group of skills for all classes. Or there should be a comparable set for every skill available to the class.
For example:
“(2) Set: Your skill performance is increased by 20%.”
“(3) Set: The duration of all time-based skills is increased by 25%”
“(5) Set: All Passives gain an additional 100% of their stated effect.”
Any class could put on a set like that, with any skill and passive equipped, and enjoy a massive gain in performance.
Or, thinking of defensive skill groups only:
“(2) Set: Your Defensive skill performance is increased by 20%.”
“(3) Set: The duration of all time-based Defensive skills is increased by 25%”
“(5) Set: All Defensive Passive skills gain an additional 100% of their stated effect.”
Or, thinking of offensive skill groups:
“(2) Set: Your Secondary skill performance is increased by 20%.”
“(3) Set: The duration of all time-based skills is increased by 25%”
“(5) Set: All Offensive Passive skills gain an additional 100% of their stated effect.”
Any class could put on a set like the two above, with any skill and passive equipped (of the bonus categories), and enjoy a massive gain in performance. In addition, there could instantly be 5 new sets for each class that would make the player of any build happy!