Making the game harder, and i dont mean like diablo where things just hit harder with more annoying aoe, i mean like difficult mechanics, difficult boss fights, and unique bosses like running from the lich king in wotlk or the math boss in ffxiv.
Blizzard probably needs to break the classes down and build them up from scratch, or just drop the spec tree all together and give them special mechanics where say a mage has to balance between fire and ice and their hard nukes with long cds be arcane.
Content and story are also becoming stale so it might not be a bad idea to end it with bfa and the next expansion be 50-200yrs in the future, give us fresh story, fresh characters, and ground for fresh ideas.