Maybe new difficulty where all monsters (bosses, uniques, white mobs alike) will have randomized Immunities, or Dual immunities will make it impossible for solo character to finish. To be more specific how I tought about it: regular Fallen in act 1 can be either Physical, Magic, Fire, Cold, Light or Poison immune. Or 2 of those immunities together. And regular Fallen standing next to him will have different 1-2 immunities.
That for sure will make multiplayer group of players a neccessity.