I was considering adding spell focus (cold), (fire), etc, to the list of valid spell focus choices in my house rules. This would enable a caster to choose a subtype of spell and apply a spell focus to all spells of that subtype.
Nah, I have an Energy Focus (or Elemental Focus) Feat, which stacks with Spell Focus, should the spell in question be both from the appropriate school and have the appropriate descriptor.
Be careful if you with that if you allow Energy Substitution or Energy Admixture from T&B. With those a person could change all of their elemental damage dealing spells into that specific type.