Actually, on my last solo playthrough with a sorcerer, I decided she was scared of fire and so she never uses fire based spells.
I love stuff like this
I'm doing a playthrough now with Wyll in the party along with Shadowheart, Gale and an Elven Ranger whose favored enemies are aberrations fiends etc. I have Wyll mostly avoiding using any obvious Warlock spells at this point as he is trying to hide his true nature in fear of rejection by the other members (after all he does bill himself as the BLADE of the Frontiers). Obviously it will all come out eventually but for now its one way to make yet another replay a little more interesting.
