difficulty incentives, like each encounter won on dark gives 20%+ ec and exp.
with the game's structure it might be best to not focus on rewards for explicitly being locked into a difficulty the entire playthrough, instead something like achievements for "you beat (unique encounter on dark) depending on how ok achievement bloat would be, could also have achievements for each unique encounter for every difficulty, same thumbnail but different, replicated borders for less work, ie dark has fancy, story has crappy (like tales of maj eyal). avoids unique tangible rewards so doesn't punish people but gives bragging rights incentive to try challenges and take more time with the combat system, been some time since it was talked about but if the changes go through to also make dark more difficult or challenging it'd be fitting around then to add something like that.
additionally, since it's on multiple formats, it would probably be an idea to have achievements also unlock independently within the game itself. ie after fulfilling the requirements for one, have an additional gallery tab where you can flip through a list with the thumbnails and borders, so if someone is or isn't on steam, they can still unlock them and have more incentive for alternative outcomes to hopefully boost player retention.
