I've noticed that every Wizard NPC in the game has a kit already. This is excellent, it makes them more interesting, it makes them less redundant, and should be extended to the other NPCs.
Mostly this is intended to spice up unused NPCs (like the myriad non-Imoen thieves in the game). I personally like the idea that NPCs can do illegal things, so I'm going to come at this issue from that perspective. For example, Montaron would be perfect as a Fighter / Assassin, even though this would not be a legal choice for CHARNAME.
Ajantis is a Paladin of Helm, like Keldorn, so make him an Inquisitor for continuity.
Alora: don't know her very well, I always have a good Thief by the time I meet her. Any ideas to make her worth consideration? Bounty Hunters are pretty hot around level 6, but that doesn't seem to suit what little I know of her personality.
Branwen: Priest of Tempus, obviously, but what does that mean? I'd give her two dots in War Hammers (illegally) at the very least.
Coran: this guy wants to go hunting big game, so making him a Fighter / Bounty Hunter might make sense. His personality makes me think Fighter / Swashbuckler, though. Tough choice, particularly since he's already pretty darn good just as a Dex 20 F/T with 3 dots in Longbows.
Eldoth, who talks about how stupid everyone is -> Jester.
Faldorn's lousy ability scores make it look like she's already suffered the harsh rituals required to join the Avenger sect of Druids, and her personality sure seems to match the vengeance-driven archetype.
Garrick has been known to comment on life being glorious, so he's probably been raised on tales of heroics and such. Make him a Skald, and he'll be able to solidly contribute to combat in spite of his uninspiring ability scores.
Imoen's personality feels to me like a natural Swashbuckler, but leaving her a vanilla Thief so she can eventually dual-class to a specialist Wizard would be fine, too. (I know it's not currently possible, but I'm hopeful for the future.)
Jaheira is fine as-is. Druid/Fighter is a strong vanilla combo.
Kagain might work as a Berserker, but he's also fine as a vanilla Fighter. He's already pretty special because of his Con score.
Khalid ... I can't think of a good kit for this guy. Wizard Slayer, so you have an excuse to not give him any good loot? Kensai, so you have an excuse to send him naked against some basilisks? EDIT: Fighter / Mage ("look at his 15 Int"), suggested in this thread. I'm thinking Fighter / Diviner?
Kivan is a good fit for Archer or Stalker. I'm leaning towards Archer, since there is already Stalker in BG2. (As a side note, Archer is one of the few specialist classes that I don't mind doubling up in a party.)
Minsc is pretty great as-is, but I'd like him to get extra daily Rage uses somehow. Maybe make him a custom kit ("Rashemon Ranger") which just gives him Rages like a Barbarian as he levels up, and of course the kit is unavailable to CHARNAME.
Montaron, who talks like he's going to assassinate you and other NPCs -> Fighter / Assassin.
Safana is the third Rogue who might be a decent Bounty Hunter, but who I don't usually use long enough to gauge. EDIT: "Her backstory screams Swashbuckler", that also sounds good to me.
Shar-Teel has "Berserker" or "Barbarian" written all over her. (She's unfriendly so I can't get close enough to read which.) EDIT: Some people like to dual-class her to Thief, so Berserker seems better.
Skie: I have no idea, I never use her. EDIT: Some rumors indicate an upcoming Shadowdancer class might be added, and she's got the dancer angle covered.
Viconia needs very little, as she's already got plenty of things which make her special. Perhaps add the spells "Blindness" and "Invisibility 10 ft. radius" to her spell list, and give her a custom class name ("Priest of Shar").
Yeslick may be fine as-is. I haven't used him long enough to have a strong opinion.