How about new quests where only outfits or only S1 heroes can be used. Or something like 5 stages only verry fast mana heroes - 5 stages only fast mana heroes - 5 stages only average heroes - 5 stages only slow mana heroes
I love a challenge so would be happy to see something new and difficult but this might also be too much for some of the player base.
Just one gamer’s opinion
Game Well 
I think something like this would be great, but what would the reward be? I would def go for the season heroes only. Maybe a new way to get some summon tokens prior to their event.