Hi guys,

In my next campaign, I want to switch things up a little bit by making all humans able to fly.  Like literally I want to give the human race, a natural flying speed

Basically, I want to say that humans are born with wings, that develop into full wings as they go through childhood.

So they start off not being able to fly at all, their wings develop and they can fly a little, then by the time they are young adults, they have have fully developed wings and can fly properly.

How would I compensate for the most prodigious race in the game, being able to fly though?

Maybe treat them all as variant humans, but instead of a feat, they get a flying speed?  

Like with Arakocra, I wouldn't let them fly in medium or heavy armor.

Or, simply make Arakocra the dominant race in your world and eliminate humans entirely.

Let more races than just humans fly. Say all non-aquatic humanoids. 

Just let it happen, then model the world appropriately.

Flying humans are the dominant race. Anyone who questions that rule ends up facing squadrons of dart- and bow-armed fliers. No city-state is safe. Either the flying soldiers massacre your troops or invisible flying assassins drop poison and plague into your water sources.

Elves and Dwarves maintain their lands by retreating into forests and caves, places that the winged troops can't reach. In response, slave armies of Goblins, Gnomes and Halflings regularly invade Elven and Dwarven lands.

Demihuman adventuring parties cross the lands, desperately looking for some Old Magic that will help them in their struggle. 

Give most species flight, for Aarokocra (if they are in your world) give them something like additional skills learned (since they have a short life maybe they are fast learners).

It would drastically change the culture. There would be no more walled cities, they would primarily be built on cliff tops or in trees. Towers with an entrance only at the top would be in vogue.  As Greenstone Walker said, most non-human races would be subjugated.  Most farming and earthbound labor would likely be done by slaves.  Many monsters like ogres that humans typically run in fear from would be easy prey for even low level humans.

Aarokocra have a very low birth rate, but if you make the most populous race much more powerful it throws the entire balance of the world off.

I imagine that a lot of monsters and magical beings would have adapted either the capacity for ranged combat, or some form of defensive upward facing shell, thick hide, or limb motion (Assuming that this is a setting where winged humans have been the norm for an extended period, not a recent spontaneous change) With a potential difference in humanity being a very different apex, I'd assume that other creatures would evolve differently to compensate.

On the topic of adaption of other races however, what about half humans? Do half-orcs, half-elves, and other such races gain similar wings, lesser wings, vestigial wings, or simply remain as is?

For similar logistics, how large are the wings at full growth? If they are significantly sizable, then human architecture would likely have to be adapted to compensate, with doors/corridors being wider and chairs might be a bit of a nightmare. Armor would be a problem, as even if the wings are capable of carrying the weight, there would likely be a weak point to the armor between the wings. A lot also depends on the flight speed, and weight capacity of the new wings. Can they bear the weight of armor or are the limits similar to Arakocra? Also, how stressful is the use of these wings? Is it more tiring to lift your weight and fly than it is to simply walk or about the same? Can the wings maintain long distance flight or is it in spurts? Can humans maneuver well in the air, or are they a little clunky with such an non aerodynamic form? Perhaps the average human walking pace may have reduced due to over reliance on the wings. Maybe the winged humans have lighter bones to reduce the load on the wings, and have a lower constitution to match

Otherwise, I'd assume anti-air/air to air combat would be a much greater focus in weapon development, so maybe bows are more powerful/magical bows are more common, or maybe crossbows are more intricate and significantly more widespread. In a similar topic to non humans hiding in the woodlands/caves, I'd assume straight up underground bunkers with narrow corridors (Assuming the wings are bulky) would be a pretty sufficient way of bottle necking humans. Illusory magic seems like a good bet too, hide structures and maybe entire cities from aerial view.

For human culture, I imagine that horses would be less necessary, unless in pulling carts (Though perhaps a heavier/sturdier beast of burden would be exchanged?) and contact between settlements would be somewhat safer when the bulk of lesser beasts can be bypassed. Depending on the ability to fly with weight, places that would have been unwise to build in (High plateaus with narrow/unsafe passes for example) may become defensible human settlements. I could see them potentially becoming messengers or couriers for other races. Unless you shift the alignment of humanity as a whole however, I don't really see them enslaving the other races in a D&D setting (could make for an interesting story though)

I'd imagine that walled settlements would still be a thing, everything else still has to walk like normal including an array of nasty wildlife, but I wonder if human settlements would omit the need for gates. I'd assume that the walls would generally be far more heavily guarded with bowmen however, and probably include casters with spells like control wind, earthbind, or other ways to ground airborne attackers. Maybe take the potential crossbow innovations to add miniaturized ballistae to defensive emplacements? Probably throw in some roofed wall sections to take cover behind as well. Similar ranged combat adjustments to any military or guard style forces would probably be pretty logical too. Long ranged holding or paralyzing methods could be a nice way to drop the winged humans and maybe deal with weaker ones outright, though I wonder if an arrow to the wing would have similar effects

Think about defenses. In modern cities defenses are not as vital consideration but for fantasy cities that would be huge. 

In most places walls force people to gates creating choke points. So how would flying societies handle this. Well owls handle this with burrows, hollow trees, digging burrows and so on. So cities, citadels, and castles are going to be more like Navajo pueblos built along and into cliffs. Just like walls and gates are choke points the cliff denies angles of approach, and caves, tunnels, and hollows force traffic to defendable places. 

In places where there is nothing overhead networks of canvases, ropes, and awnings limit approach. 

Slate and tile rooves which are more fire resistant are much more likely. Seeing as from above is always an avenue of attack. 

Nets, and bolas are going to be much more common. Huge nets are going to be weapons of war launched by ballista. 

I'd set the whole campaign in The Underdark or the elemental plane of water... but I am a jackass DM so it's probably best to ignore me lol

Quote from Welsh_Grifter >>

I'd set the whole campaign in The Underdark or the elemental plane of water... but I am a jackass DM so it's probably best to ignore me lol

 You can still fly in the Underdark, some of it is truly enormous on a surface level scale, it just happens to not be the surface.

Oh, of course, but there's lots of cramped little tunnels to mess with them too ;)

Giving them hurdles to deal with is part of dnd in my opinion.  Oh you flew ahead of the group to scout, better watch out for the enemy archers...

I don't think Humans would be the most prodigious race if they had the flight ability via wings.  Flying takes a lot of energy.  So instead of those calories going to brain development, they are going to flight.  Also, think about what a toll that pregnancy takes on a flying human.  Eagles get by with laying only one or two eggs.  Late stage pregnancy is a serious burden on a human family.  It's not only the added weight, it's also how that changed weight distribution throws off the woman's sense of balance and the increased appetite she gets.  So that's 2 to 3 months where flying might actually become dangerous for her.  Do you see where I'm going with this?  All these factors would cause flying humans to have significantly lower rates of reproduction and work compared to how we usually think of human societies.