oh that's already a planned side quest. you're just supposed to gather all the diary pages first xD (there's only one in the game right now, but there should be at least 3 in total)
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loved the idea! there's already something similar planned for the main quest, but (for spoiler reasons) it'd be nice to have something else on top. a side quest would definitively be a good way to implement it.
btw, not sure if you've already found it, but there's a spell book hidden in the game that will teach lianna a certain lightning spell. the plan is to have at least 3 hidden spells of elements aside from fire (she only learns fire and dark spells from levelling up, as those are the ones she learned from her master). none of them is particularly overpowered though, and only serve to give you situational advantages (lightning having a chance to stun the enemy and ice projectiles dealing physical damage), so a secret OP spell unlocked in a side quest does sound perfect.