The Focus Mage picks one school of magic as his primary school and gains all cantrips and 1st level spells at first level, gaining all 2nd level spells from that school at 4th, then 3rd at 6th, and so on. The Focus mage can cast spells as often as he'd like but there is a 'cool down' period based on the spell level where he cannot cast any magic at all. If the Focus Mage chooses to push himself and cast anyway he takes a small amount of subdual damage and the cool down period resets, and is increased for the additional magic cast.
This class makes for very useful but highly specialized spell casters. The versatility comes in the form of talismans: at 1st level, and every two levels afterward, the Focus Mage gains a mundane item that he can use to channel a specific spell from a different school than his primary. This is useful for Universal school spells because there are so few of them that it would be silly to take the school as your primary.
The only difference in the mechanics of spell casting are that the Focus Mage requires a focus (hmm, you don't say) a staff or wand, a crystal, basically any mundane, simple item that he dedicates to that purpose, that without, he is basically powerless. The second difference in mechanics is that all spells cast by a Focus Mage gain a verbal component if they don't already have one.
I've play tested the class once with a Gnome Illusion specialist and had a single encounter with a villainous Gnoll Necromancer. It worked reasonably well both times, it means you need to have a little more of a plan as to when and where you cast spells but you'll never run out. You can literally knock yourself out with the power coursing through you but being an unstoppable thaumic monstrosity doesn't come cheap
The full class is attached, I'd love some feedback. Play test it, tweak it, let me know what you think.