Gale - Agree on Transmuter
Astarion - Assassin
The Assassin subclass is, ironically, not really about killing people. It's about subterfuge. Masquerade. Blending in with your victims, sharing space with them, breaking bread with them, raising no suspicion until an opportunity presents itself. Assassin subclass features are centered around disguise and acting. If I'm not mistaken, this is basically what Astarion's been doing for centuries at his master's behest.
Lae'zel - Agree on Battlemaster
It's worth noting that githyanki warriors tend to blend magic with swordplay, and Lae'zel claims to have "dedicated as much time to her studies as to her training". That said... her Intelligence is 11. None of her dialogue implies even a passing interest in magic, but she is a seasoned warrior who's commanded others before.
Halsin - Definitely Moon
Minthara - Oath of Conquest(?)
Wishful thinking aside she's clearly Vengeance. Conquest would be so fitting though, since commanding through fear and spreading terror to the Absolute's enemies is what's she's all about. One can hope.
Wyll - Pact of the Blade
Naturally. Although his whopping 13 Dexterity makes him the saddest bladelock in Faerun. Really wish we could respec him because right now there's no reason not to go for a free imp at 3rd level.
As for the other announced companions, we don't know their classes for sure, but I'll hazard a guess:
Minsc - Some unique Ranger subclass with Rage
Jaheira - Circle of the Land Druid/Battlemaster Fighter
