Some kind of World event could surely help D3. D3 is very boring due to no trading, and easy drops. D2 sucked in part because of trading (G-Rush, Uber leveling, buy your gear on D2JSP then follow somebody's baal bot, hope you snag some drops and sell on JSP).
I dont know what the answer is but it lays someplace in the middle. D3 seasons get stale fast, pretty soon you are grinding hours just hoping for another legendary with slightly better stats, the stuff you do get is useless, cant sell it, cant use it so you scrap it. Part of the lure of D2 was getting a legendary to drop and it could be for another class you want to build or something to trade or sell, back when stuff was actually rare.
Im not sure where D3 went south, I see they were trying to avoid some of the pitfalls of D2 (rampant bots, massive cash based economy outside of game) but instead they created a game that goes south fast after every season is a few weeks old.
They need to add somekind of Rare World event, some rare drops, bring back runes or something. I try to get excited for each season and that fades quickly everytime.
They got rid of everything that made D2 go bad, but that didnt make D3 a better game. I dont know if there is an answer, maybe its time to stop beating a dead horse and move on