So in a game i used to love called rift ( went p2w sadly )
there was a thing called chronicles, which are dungeon things but its for 2 payers or 1 player. ( in rift you had to be in a pre made party, that was the worst idea possible.) i really liked it because it gives more oppertunities for social stuff. in a group of 5, its too many ppl to socialise, its like a classroom at school. if you were to be 1 on 1 with another kid in your class, you would become freinds, but theres 30 kids in a class. so adding 2 person dungeons will cause people to be forced to have social interaction.