Companions that could be added based on missing alignment / classes (including new class potentially added):
Artificer - Barcus Wroot. As mentioned before, he basically already travels with our party and can just be allowed to participate in events starting from Grymforge release - get alongside us to LLI, save Wulbren from Moonrise Towers himself and so on.
Monk - new NPC, since we pretty much never meet them excluding the ghost and the ghost is incapable of fighting (as he needs a body to act on his own like shown in act 3). That could be for example a male githyanki from Creche Y'llek - the location is fitting for him and he could have cool story interactions with Lae'zel (being more peacefully aligned compared to her aggressive approach), Voss and especially Orpheus, who is a monk himself. I don't suggest Orpheus himself since he is capable of joining us way too late to become a companion.
Bard - Alfira. She could be not recruitable in Act 1 when she is busy with her new song, but could be recruited in Act 2 in Last Light Inn to help save other tieflings and then become a permanent companion. She wants to write a song about us anyway (for good playthrough) and travelling with party directly would be quite helpful. As for the Dark Urge nuances, people have already found a way to address their situation.
Sorcerer - that is more tricky, since we almost never meet any during the playthrough and the one we meet is both a bit crazy and busy with the last rites. I still suggest recruiting Wild Magic Sorcerer Delina somewhere in Baldur's Gate - she is an ally of Minsc and likely would not mind helping him and his allies save the city.
Good aligned Paladin (compatible with Halsin) - Zevlor. After events in Act 2 he gets separated from his crew and searches for a new purpose and that purpose could be helping us deal with Absolute directly.
Evil aligned Druid (compatible with Minthara) - Kagha. While she would not be fond of you destroying the grove, she could become recruitable if her plot is exposed before the siege happens and is banished from the grove and then your character finds her and convinces her to travel with you.
Alternatively on Kagha, telling her you know about the Shadow Druid stuff, but completely stand by her side on that, then kill Halsin and one way or the other evict the Tieflings, other shadow druids might take over and she herself goes on a journey with you, perhaps to spread shadow druid influence?
