The solution could be quite simple if one would think about it. Here's some food for thought:

Dungeons could be made non-linear and with multiple rewards. They'd have to pull some ideas from other games (like wow), but it'd be possible.

Have multiple branching paths that overlap in places, and several bosses scattered throughout the dungeon. Not just the ffxiv standard of 3 bosses per dungeon...but maybe like 6 or 7 bosses in the dungeon. In order to get to a boss, you may have to go down path C which requires you to do something to 2.0 toto-rak and the little balls of light. You gather them, you get access to that boss. Said boss drops a chest that pulls it's rng loot from the gear loot table for that dungeon. More bosses, more loot.

Of course, that would just make people take the straightforward path to the endboss. Which is why one little dungeon mechanic would need to be added. RNG DOORS!

Imagine you start the dungeon, and you go forward. You immediately find yourself with 3 paths before you. Except that this instance, the dungeon loaded with paths 1 and 2 closed. You have to go down path 3 which takes you a certain way. Next time you enter the dungeon, doors 2 and 3 are closed. Guess what?! You're going down a new path! You'd technically be able to fight all the bosses in the dungeon if you EXPLORE (OMG), but even if you decide that you want to try and just rush it, your rng doors will make sure that you'll be taking different paths each time.

i.e. you'll fight 3 bosses before the end boss, and there are 6 bosses not including endboss. One run you'll fight 1, 4, 6. Another you'll fight 2, 3, 5. Another you'll fight 1,2,4. Ta-da! You're dungeon just got interesting. Why? Because there are multiple paths, and which paths are unlocked are randomly determined the instance is loaded.