Basically take existing all the HW dungeons but give them a difficulty selection (+2, +3, +4, etc.) more HP/Damage and a range of additional modifiers (trash mobs leave damage puddles on death, trash buff enemies when killed, all mobs stack a debuff that absorbs healing, etc.). Rewards scale with difficulty and more rewards for faster clears (within time limit is one chest, 80% of limit is two, 60% of limit is three). The modifiers rotate weekly, though from a set list, and all the current dungeons are available to be chosen randomly for your dungeon so it's some variety with that.
It's still infinite grind content but it's also with scaling difficulty to it instead of farming a currency. It's imperfect with the reward structure still but it's hard to not expect communities to tend towards the path of least resistance options no matter what (aka, fastest dungeon only, rip longer / more annoying dungeons).