Yay for noncombat companions and (non-)combat animals!
Nay for undead and werewolf party members. I like the design of mere humans against the darkness, it is what made X-COM strong. Had you been able to field psionic Greys and recruit Chryssalids, the game would’ve been powertrippy 90s shit.
I was thinking about vampirism and lycanthropy, but I’d implement them as very rare events that basically cost you a Brother. Think “Organ Trail”, you’d basically be forced to decide whether to a) keep them along until they transform and attack you, b) put them down, c) let them go (in the knowledge you’ve released a dangerous monster into the world) or d) subdue them and try to find a cure, which if I were the game designer would probably never happen, but could lead to interesting events, e.g. villagers finding out one of your men isn’t what he seems to be.