Shins remark is a very pertinent one. Some less used focuses could really have a bump in use
Currently the top focuses, in my opínion, are:
-Necromancy - Horrids, Death Spells, Energy Drains: The top killer
-Transmutation - Stonehold, Drown, Flesh To Stone: The top disabler
Second, in a (very) lesser extent:
-Evocation - high elemental damage, but reflex save makes them useless against dexers
-Enchantment - disabling spells that are killers to mid-20s but near useless at 40 and also mind-affecting
Now, what other focuses offer?
-Illusion: Actually gotta run some tests, just realized a few things about it
-Conjuration: Flame Arrows, and?
-Abjuration: what?
-Divination: ????
So, we could spice things a little more for these ununsed focuses. Let's say:
-Conjuration: focuses could add to summoned monster levels. For example, a Spell Focus could add +2 to hd of all summoned creature's
-Abjuration: it could add a bonus to caster levels when casting a dispelling spell or a defensive spell. For example, a Spell Focus in it could make a 20th lvl sorcerer cast premonition and Mordor disj as if he was a 22 lvl one.
-Divination: See Invis, True Seeing, Clayrvoyance, Amplify(I think). Maybe augment the power of those spells, say, Clayervoyance adds 10+2*focus in listen/spot, same going with amplify. It could add 2* Focus duration in round to TS, for example.
Anyway, we can play with many things here