Well, according to what you posted earlier, you are already a mage/adept...those are your two core classes. (first level)
second tier (second level) class=sorcerer; this is what you have already.
For your next second tier class you can choose Paladin, Celestial or the Eastern house/adept class of Monk.
For your third tier (third level) class you now have the choice of Wizard, Crusader, Stargazer or Shaolei master, depending of course on what you want for your character and also what your choice of your other second tier class is. Keep in mind you will only be able to choose ONE third tier class. The possible builds are as follows:
mage/adept sorcerer/paladin crusader
mage/adept sorcerer/paladin wizard
mage/adept sorcerer/monk wizard
mage/adept sorcerer/monk shaolei master
mage/adept sorcerer/celestial wizard
mage/adept sorcerer/celestial stargazer
It might seem a little confusing, but read through the classes (the link that POISON and I posted). You will see how classes are gained depending on earlier classes chosen (dependency chart) and you can also see what learning bonuses and skills each class has. If it was me, I would choose wizard as my ultimate goal, but of course that is up to you.
Keep in mind the different skills each class gets, and see what interests you. I always liked adding "Monk" for my celestial magic characters as you get some nice learning bonuses. (Remember, go to the link given and read over the chart!

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