I'm mainly sticking to the default paths for most of the characters; I made Yuna and Lulu learn the basic White and Black Magic spells (Cure, Esuna, Fire, Blizzard, Thunder, Water), Tidus and Wakka are going down their normal route, and I'm not sure what to do with Kimahri yet.
Also Yevon bless the Extract Mana / Power / Ability Spheres abilities. I've been running out of spheres left and right.
