Or as if you said 'not for free' then how about feat which allows me such customization? Not do it on fly and decide between element types but simply feat which states; 'Any spell sorcerer picks becomes fire type spells and changes any saves to reflex saves'. It's kind of handicap feat but it allows focus to single elemental school and so focus feats around empower fire spells and not 'locked' with choices what you made with feats and pick spells which have fire type. But pick any spell and it becomes fire spell.