Bumble, if you want to make your party "a little stronger and more fun", I think you need to choose the NPCs who have the best dialogue, and you should have each of your party members concentrate on their strongest skills.
For example, if Mazzy is in your party, use her as an archer, since that is where she excels.
I'm guessing that your Kensai main character is your strongest Fighter, but you seem to have a lot of strong fighter-types in your party: Minsc, Keldorn, Cernd (shapeshifted to a werewolf). You could replace one of them with Korgan if you want your party to be even stronger in melee, but I really don't think you'd gain all that much, since you're not weak there in the first place.
If I were you, I would add more spellcasters to your party. And I wouldn't choose spellcasters on the basis of their melee skills, which would be missing the point in having them in the party. Who cares if Viconia isn't as good in melee as Cernd or Jaheira? Look at what she can do to undead. If you don't use her in melee all the time, you can actually have her cast spells in combat, which Jaheira and Cernd can't do if they're busy hitting their opponents with melee weapons.
I'm glad that there are people who come to the defense of Aerie's prowess in combat. If you think she's "useless", then you have no idea what she's capable of at higher levels.
But if you want a "Cleric" in your party, then you can keep things simple and choose a character who's a Cleric and play her (or him) AS a Cleric and let her excel at what she does best. Viconia would probably be the most interesting to you, especially if you pursue the romance with her. I don't think she's as strong as Anomen, but if you can't stand Anomen and Cernd, you could replace Cernd with Viconia. Jaheira is a strong character, too, but I don't think she's all that much fun. Dramatic, yes, and sometimes interesting, but not fun.
Personally, I don't think that Thief skills are an essential part of the game. Imoen has enough Thieving skills to get by (though just barely), with the exception of backstabbing, which you might as well forget about. You should let Imoen look for traps and unlock doors and chests. But for the most part, you should play Imoen as a Mage. She's pretty good at higher levels.
But I suggest that you add a second Mage to your party, and you might as well pick the best. That means Edwin. Since Keldorn's best abilities besides melee combat are Dispel Magic and True Sight, you can replace him with a powerful Mage. Personally, I don't like Edwin (I kill him the first chance I get and steal his amulet, ha-ha), but Edwin will (quite unintentionally) add a lot of laughs to your game.
And speaking of adding laughs to your game, you could replace Imoen with Jan, since she runs out of dialogue after you rescue her from the Asylum (although her dialogue picks up again in Throne of Bhaal). Jan can handle your thieving needs a lot better than Imoen, as well as help you improve your vegetable sales.
If you replace Cernd with Viconia and replace Keldorn with Edwin, that unbalances your party and makes it considerably weaker in melee. Personally, I don't consider that to be a problem, since all you really need to beat the game is one Fighter, one Cleric, and one Mage. But if your playing style is dominated by melee, then you need to strengthen your party's melee power. In that case, I'd replace Mazzy with Korgan. A Lawful Good character doesn't belong in a party like yours in the first place (the only reason such a character would travel with you would be to keep an eye on YOU, not just your enemies), and in my opinion, archers just aren't as effective as other Fighters in the advanced stages of the game. I like Mazzy, but if you want more power, lose the bow and pick up a sword.
So here's a party you might want to try:
Kensai
Minsc
Viconia
Edwin
Jan
Korgan
Notice how the only three Evil NPCs in the game are all desirable party members? Thank goodness you have Minsc and Boo in your party, whose Goodness matches the Evilness of all three of the Evil NPCs combined.
