I personally am not out of content yet from the core set but have been preparing ways in the back of my head for the eventuality of additional challenges.
Team Covenant used "Super Expert Mode" on one of their recent (maybe yesterday?) streams. In that mode they flip 2 encounter cards per player during the reveal encounter cards step of the villain phase. Nice and simple.
For lack of a better name my proposal for a "Nightmare Mode" includes all Expert Mode setup instructions: use of the 3 expert encounter cards, and villain levels 2 and 3. Additionally, you'll start the game against level 1 villain in addition to battling levels 2 and 3. Also, the players' nemesis encounter cards will start the game in the encounter deck. When you reveal "Shadows of the Past" you'll search the encounter deck and discard pile for the character's nemesis and their associated side scheme and put them into play. The surge would trigger if the player that drew Shadows of the Past already was engaged with the character and the side scheme was already in play. Again, simple with only altering one card.
Anyone else put any thoughts into alternative difficulty modes?