We don't have any plans to make FF a toggle. Our approach with difficulty is to add or remove enemies to change the tactical considerations of encounters and to hide or reveal information based on user preferences. FF is just a part of how some spells work and I don't think it's a good idea to make that something the player can toggle. It changes the values of those spells too much, IMO.
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When FF gets turned off, the effectiveness of all spells that could have FF goes up significantly. If a subset of AoEs are to be markedly powerful compared to alternatives, FF (or some equivalent) should exist as a drawback. This allows the player to choose between using powerful friend-or-foe AoEs, less powerful foe-only AoEs, and powerful targeted spells. When FF gets turned off, that middle category largely becomes obsolete and even the targeted spells logically get bumped down a peg.
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Yeah, FF is definitely a deciding factor when I'm picking a spell to cast. There are a few foe-only ones that don't quite pack as heavy a punch as the friend-or-foes, but they are the tactically safe option.