A lot of people are linking the "Variable difficulty" with the number of players but I took that as a different aspect. I thought that "variable difficulty" was separate from the "1-4 players" (i.e. there would be some sort of variable difficulty even between 4 player groups).

Maybe something similar to FF7's arena mode, where after certain points in the dungeon a slot machine shows up and somehow debuffs the party or buffs enemies and those stack up as you go (similar to the effects in PotD/HoH)? In FF7, the "worse" the debuffs, the more rewards you got for winning.

Or, just simply the same dungeon that gets more and more difficult as you increase its difficulty.

It seems like it'd be silly to just add something that was just for 1-4 players because that's kind of what PotD/HoH are for.