2) Add mod support. WC3, D2 and WOW supported player made content and/or addons. This was a huge factor in their longevity in my opinion.
D3 is the black sheep of the blizzard family. Giving fans these two things would be a HUGE step in the right direction. My third step would be to drastically speed up content/patch/fix delivery. Especially when it comes to fixing itemization and adding items to spur the creation of interesting builds.