Well, the simplest option takes a page out of the wizard book from Pathfinder, so not so much homebrewed as backward conversion: allow your evocation spells to convert the type of energy damage they do. For example, a fireball becomes a lightning ball instead. Effectively the same spell, just doing electricity damage. This way you can use other evocation spells and just changed the the name and damage type.

I'd say this would take a feat, but considering that were somewhere around level 10 you should have one or two to spare.