Make the end-game related to the storyline. Rifts have just always felt out of place for me. I defeated the prime evils and now their minions are stronger than them in these rifts?
Allow the players to choose their gear for each slot. These set items and their accompanying big damage boosting legendaries feel so restrictive. You have to use certain items in order to use certain skills, it's terrible game design.
Allow the player to determine the power of their skills as they level up (like D2 skill tree). The removal of the skill tree has allowed some runed skills in D3 to be the outright best choice. All they had to do to remove the restarting due to skill tree error in D2, is to add a respec option, not remove it completely!
Get a new development team. It is clear the current team doesn't care about the legacy of the Diablo franchise, and just want to implement their own way of things.