This is something that was done years ago by a few games …. City of Heroes, Anarchy online and even to some extent Everquest 1 with Lost dungeons of Norrath. this idea is already implemented in other game genres such as diablo, torchlight 2 and Path of exile.
the basic premise is that you go into a dungeon and it randomizes the layout, the mobs and the bosses. the Dungeon then would scale the amount of mobs depending on how many people join with you. You can enter these dungeons solo or all the way up to a full group and the dungeon would scale its difficulty to your numbers.
bosses would be random with even rare bosses popping up sometimes (there would be a greater chance to see one of these rare bosses if you group with more people). And like the bosses Exotic/Ascended gear would have a chance to drop in these dungeons again with the chances for exotic/ascended gear to drop being greater depending on how many people you have joining you, however even solo there is still a chance for loot to drop.
if added this would give near infinite re playability to the game.
