[deception5]

My personal choice would be to stick with a caster secondary as this would pump your int/mana (from masteries alone) and allow you to continue to focus on int.

Storm would be a strong choice with lightning, squall (reduce elemental resistance/cause lightning damage/reduce enemy damage), storm nimbus synergies, and especially eye of the storm (which will significantly boost your fire damage as well).

Nature would be good for plague (reduce resistance)... well mostly for that although heart of oak is nice and there are some other nice things.

Spirit could be good as well for the outsider (particularly where you are looking for help in boss fights as he is the strongest summon), not to mention the lich king, enslave, dark covenant/unearthly power, or even ternion.