So I've gone through the game once and decided to do it again but I need some advice.
I'd like to go through with mainly different people and a different class NPC but I have no idea what. Normally I play the hack people to little bits/ blast them to little bits style, but this time I want my main character to not be a tank.
So, for my first run I used
Me (Female Human CG fighter)
Keldorn
Viconia
Haer'dalis
Imoen
Aerie

For my new party, I know I want:
Me (CG and male for romance with
Jaheira
and my two favorite characters Keldorn and Viconia.
So I need a class and two new people to complete my party. Maybe I'll drop one for Imoen, though that would kind of defeat the point of using different people.

I'd include Jan Jansen for your thief/spell caster. Give him the robe of vecna, and he rocks. The other NPC could be a tank of some sort, e.g minsc.For your P.C class, i'd recommend either a ranger, or a spell caster of some sort. If you played a fighter before, go for a sorcerer.

If you want to try some new characters I would say that you do a thief PC,
Jaheira,
Keldorn,
Viconia,
Edwin (if you do not mind an evil character, i thought his dialogue was hilarious)
Imoen if you want her for story or I would go with Mazzy for a new character. I hear she has good dialogue with Keldorn and makes a good Archer

I have three options:

Option 1: Play a sorcerer. However, if you don't want Imoen, you're still going to need a thief, so that leaves Nalia (not a particularly good thief) or Jan Jansen, who also would serve as a backup mage.

Option 2: Play a PC thief. It eliminates the need to take on Imoen or Jan, but you are now going to be out a mage, and here I'd recommend either Nalia (who is a good mage even though she isn't a good thief) or Edwin, who is easily the best mage in the game. Your last member can be anything you want - try out Cernd or Valygar for something different.

Option 3: Play a multi-classed mage/thief. It kills two birds with one stone. Then you can take on additional mage if you want, or take the Valygar option. This option may be the best because it gives you the most versatility for the remaining two members of your party.

Not to mention that a M/T is one of the most powerful classes in the game. Once you get mislead, the battles get easier and easier. Imagine. 70 points a hit, twice per round (a la Belm). Nice!

A sorcerer is an excellent change after playing a fighter, indeed. No matter what many people say, there are too many traps in the game to go through without a thief, so Jan, Yoshi or Imoen are your choices, too.