In my opinion, it'd be nice if they added difficulty to dungeons. Even if it were in what some would call a, "tedious" way.

Make a new set of dungeons. Call it, "Long Boi+". Have the dungeons with twists, turns, minor puzzles, and have it so that either the timer is shorter or the content takes up most of the 90 minute default. Maybe make the bosses on par with normal mode raids. Make the rewards gear that's 5 iLvls higher than normal mode raid. Have it come out a patch or two after the normal mode raid of each tier.