Personally, I would expect elementalists to be sorcerous types, but if you want a learned/clinical approcah, how about basing it on the wizard, simply offering 5-6 additional wizard specializations, one being the 'general' elementalist.
Instead of prohibited schools the elemental specialist would be prohibited from the opposing element and its related spells on the secondary list, or (in the case of the general elementalist) prohibited from all non-elementaly spells outside the secondary list.
The second restriction will probably turn out to be the strictest which allows for a small extra benefit. I'd suggest allowing any elemental spell to be prepared as the bonus spell whereas the focused elemantalist must choose one using his chosen element.
Familiar: If the elementalists are made as wizard specialties as above, a restriction could be imposed that any familiar taken must be elemental in nature. This would in itself delay the aquisition (sp?) of a familiar until around level 6. This restriction may be offset by allowing elemental specialists elemental familiars as per the feat.
Wood element: As far as I remember, the wood element represents life, nature, growth and vitality in the Earthdawn RPG. A similar definition could be used here, adding a lot of druid spells to the list of elemental spells. However I'm not sure what the 'opposing' element should be. Some sort of stillness/death/negation grouping would seem to be the opposite, but that's not really elemental in nature.
