There are lots of different possibilities here.
If you are you going the complete caster way I would probably suggest Dream, Spirit or Nature.
Dream if you want another form of attack that can stun your opponent in place. I have also built a Dream/Spirit caster and have to say that Sleep from the Dream mastery is the most under rated spell in the game. I have boosted this to the max and find I can put a large mob of creatures (not uniques or bosses) to sleep for up to 12 seconds....by this time pretty much you could probably go out get your self a drink come sit back down and then make a decision as to what spell you plan on finishing them off with!
Dream also offers the 2 distortion type blasts - one the wave and the other the nova effect....I would go for the nova one if you already are throwing Volcanic orbs at people....a nice little combo could be orb then sleep then wlak up to them and nova ripple them....while your core dweller is bashing them.
Spirit is good if you decide to use your staff damage (due to enchant) as your main damage causer this backed by Ternion can cause a stack load of damage to the enemy...also in the spirit tree the Lich King is great support for your Core Dweller....if anything the Lich King can probably tank just as much damage! The Outsider from the spirit tree is also a must for bosses...this guy is probably the best tank in the game.
Nature is great for healing your pets and for additional pets....wolves + Dryad + Core Dweller = a lot of pets....just make sure you support them well by using volcanic orb to stun enemies + eruption for mobs/bosses and the use of Heat shield on each of them when you can to make them last a little longer.
Hope this helps and keep having fun. I have managed to get my Pyromancer to level 50 now! Defeated Typhon in Epic just the other night...
...and one further thing I am finding the Gatekeeper staff that I am using (legendary staff) very good for staff damage.
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I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhäuser gate. All those moments will be lost in time like tears in the rain. Time to die.
Roy Batty (from BladeRunner)