some ideas from my side:
- there could be something like advanced spheres, which need some inferior sphere as prerequisite.
f.e. the normal healing would give the cure spells, the better restoration, cure blindness deafness, remove curse.
This option would give the opportunity to trade versatility for power and vice versa.
-- what is shadow casting? :)
- Spheres could give some special power / powers fitting to the type.
f.e. Healing could give +2 on Heal and you might expend a spell slot as swift. The advanced sphere could give the ability to cast healing and debuff spells with more range or with more potency.
The existing class features from the mages and their archetypes could make good powers as well.
- then maybe some spheres should make other spheres impossible to take, like either you are a healer or a life thief. this could make balancing easier and make things make more sense but could also shortens the variety of the system.
- a thing to think about if there is an ultimative all in one caster class, will it be free to chose a primary attribute or will every sphere have its own needed attribute. the sphere with charm person and suggestion might be charisma only while healing might be wisdom only.
on the other hand this seems like much afford. and the actual mage classes still cover all common and even not that common spell casters.