This is exactly what I’m asking. Thank you.
I’m concerned because, while not knowing anything more than what’s listed above (that’s not completely true; I know the celestial warlock plans on going pact of the tome to be a better spellcaster), since we’re using the standard array for stats, there doesn’t seem to be any need for an intelligent character. Who will do the research?
I’m concerned that, while there is a warlock that will prioritize charisma, I’m not sure what social skills he’ll actually possess.
I’m concerned that there’s nobody who will wear any heavy armor.
I’m not asking for a character that will fill all these needs, I’m trying to figure out what will compliment the above party best.
A lore bard is interesting to me. An arcana or knowledge cleric is interesting to me. A bladesinger or diviner is interesting to me. A hexblade bladelock is interesting to me. A vengeance paladin is interesting to me. Actually, most characters can be interesting to me.
I need help determining what mechanics I need to best compliment the party.
As people pointed out, from the information you gave, we have little more to contribute than you already know.
I don't know anything about Celestial warlock potential so I'll skip that. You say he's going more castery so there's your caster ranged.
The Rogue is a bit of a wildcard. They could go more thug rogue or they could go more skill and assassiny rogue. You could very easily end up with a rogue with TONS of skills or a rogue with a few useless skills.
The Barbarian is the easiest: they're your heavy hitter and your "meatshield". So you can probably cross off Fighters and Paladins (especially since you've got a holy-hybrid already).
The Hunter ranger is probably going to be more scout than damage, so count on them to cover that end of the pool. He's also your melee ranged.
The Moon druid is designed to wild shape, a lot. So they're going to be a damager/meatshield type.
What your party does not seem to have is any real control class, so I'd suggest a Bard or a Wizard for that. The Bard gives you the most versatility if things don't go the way they're looking right now, which given you're all level 1, people could switch, die or generally go different directions than expected. I'd suggest whatever Bard college gives you access to more skills as opposed to the damage-oriented side. You'll also have a good "face" if the warlock or the rogue decide not to go that route.
So there, best advice I can give based on the limited information you have: bard.
Don't worry about heavy armor. Your barbarian will cover that with a high con.