*-Add Multiple pathways, with different bosses each path. Perhaps 3-4 per dungeon, chosen at random when you enter the dungeon via Duty-Finder/Roulette as to prevent the "Go quickest route" runs. Its random unless you pre-make a group either via Party Finder or friends,, then you can chose when entering (like undersized parties). Guild Wars 2 does this with their dungeons (though you can chose them, each has a daily reward so people do them all).
*-Mix up the mob groups that spawn for each path. Maybe have a handful of possibilities each path with the general difficulty remaining the same.
*-Perhaps add "Mini-Bosses" with unique names that sometimes spawn in place of a normal mob pack, they don't need fancy smancy mechanics, just bigger/meatier version of the area mobs. These bosses would then drop a chest possibly containing a dungeon armor, or some (Unique dyes), even a chance at the dungeon exclusive minion, so forth. A rare sight that makes people excited.
*-Some people like puzzles, maybe one of the paths could have fewer enemies but with puzzles to solve to progress. I know this works better in single player games, but It'd be nice instead of just picking up X item and placing it in Y slot.
*-Spice up boss fights. They don't all need to be Square/Circle dodge mechanics. Something like Steps of Faith/A2 where some people are constantly doing a certain mechanic while others fight the boss... though maybe a little simpler for the sake of it being a dungeon and not a raid. Given time I could imagine some fights but on the spot, no. And that doesn't mean I can't think of any, SE has months of time to design fights... give me 2 months and I think I can design a couple dungeons as well...