I wanted to start a discussion of bold, risky but possibly fruitful ventures.
WHAT IF, you dual-classed your mage at 18 into rogue? THAT would be interesting. But man, starting from level 1 (THEN AGAIN, in BG2 leveling will not take long for 1-7 since you get much more exp throughout the game, but it isn't much for higher levels). Instead of rogue-to-mage, you can get one level 9 spell, a distant list of spells and by the time your rogue reaches 19 you can be equipped with a POWERFUL list of spells and some powerful rogue attributes. Sure, it'll take a massively long time, but at least it won't take as long as leveling a pure-class mage to 31. I think, although you lose some of the benefits of mage power, you can eventually have a rogue with its HLAs and some magic to boot. BUT in bg2, it is a risky venture. And some might resort to power-gaming and import into a new game or black pits and level up that way before importing back into the original save. Which is disappointing if you do so, but I wouldn't blame them entirely.
Also, Mixing a Wizard Slayer with thief. Sadly, you can't use equipment that would provide good sneaky boosts, but the magical resistance can help make up for the thief's inability to use really good armor, and if you specialize the thief in hide in shadows and move silently, you can possibly make a great backstabber, especially against mages. This is assuming it is possible (haven't tried).
If you have ideas besides dual-classing, I'd like to hear them.