Expanding on Variant Humans

Under 2nd edition rules, human characters are not seen as viable if you remove class restrictions. It's right in the DMG. The sum total of human benefits can be summed up as:

• Fairly common (no mechanical benefit)
• Generally well liked (no mechanical benefit)
• Can dual class (poor version of multiclassing)
• Unlimited access to all classes (not Comcast/Verizon unlimited. TRUE unlimited access)
• Most magic items are going to be sized for them (or elves, and half elves)

So when you want to remove class restrictions on demihuman players, you run into objections that it isn't fair. Heck, the DMG even says it:

The DM can, if he chooses, make any class available to any race. This will certainly make your players happy. But before throwing the doors open, consider the consequences. If the only special advantage humans have is given to all the races, who will want to play a human? Humans would be the weakest race in your world. Why play a 20th-level human paladin when you could play a 20th-level elven paladin and have all the abilities of paladins and elves? - Page 21, DMG, Chapter 2 PC Races - A Nonhuman World

Now, ignoring for a moment player agency and also ignoring the world building consequences (that's an entirely different rant), it is true that, mechanically, humans get the short end of the stick. However, this can be adjusted without damaging the game balance or engaging in any more power creep then 2nd Edition engages in already! And the main reason has nothing to do with demihumans - though it helps with that.

There is a strong cultural variance between groups that live in radically different situations - even apparent in the Middle Ages. Look at perceptions of Appalachia versus the cities of coastal Virginia. And there are even further divisions based on the role that an individual has within a city or rural area: landed lords vs free peasants (i.e. the economic conditions that led to Enclosure), or the "town vs gown" debates, or what to do with peasant workers migrating en masse to cities. Guilds only took in so many people, and the huge labor force cause significant cultural changes.

At the same time, these cultural variations lead to skill variations. During enclosure, peasants kicked out of common lands lost the security and safety that the old system provided. They were unable to transition their existing skills into the new market, and this lead to revolts and riots during the transition period. In Ireland, food and land seizures that disrupted the population lead to the formation of the Irish Republican Brotherhood (later the IRA).

The primary point of all this is that there are significant differences between human populations in terms of skills, temperament and inclination. What unites them is a general view from other races that humans are dirty (high populations crammed into cities; or crammed into small villages and living with the mud and animals), but also diplomatic and generally well liked. Because of that I give all humans two racial abilities:

• Peacekeepers: Humans in general are more trusted then other races. Both elves and orcs, racial enemies turn to humans at times and though humans might have preferences (evil preferring orcs, good preferring elves), there is generally no way to know at first. As such, humans always start one level higher on encounter tables: Hostile to threatening, threatening to indifferent, indifferent to friendly. This is the case until the human does something to indicate that they should be treated differently.
• Disease Resistant: In most fantasy settings humans cities are often described in typical medieval fashion compared to elven or dwarven cities: dirty, squalid, and packed to the parapet. These conditions have one natural outcome: disease. Disease often rips through human cities and kills the weak. Those left multiply and grow, having stronger resistance. Humans thus gain resistance to non-magical disease similar to how halflings and dwarves have resistance to poison and magic. They gain a +1 to their saves for every 3.5 points of constitution. The save type depends on the special attack used by the creature. Rats, for example, use the Save vs. Poison chart. If no save is allowed or outlined (i.e. rabies from a large bat) the character can make a Save vs. Poison without their bonus.[/list]

In addition to the above, I recommend the following variants based on regional variance:

Rural

Peasants and nobility that live out of the cities on estates. Humans of this type tend to favor outdoor activities and are often seen as uncultured by their urban peers. Typically they live in small villages and hamlets and are engaged in agricultural work, logging, or mining. Rural humans get:

• Ability Score Adjustment: +1 to Constitution, -1 to Intelligence. Rural humans need to be tough to survive the day to day of back breaking farm labor. At the same time, they very rarely get formal schooling and children tend to be put to work quickly, which acts as a detriment to maximizing mental capacity.
• Making the Best: Rural humans are used to selling the best of their wares and making do with what is left over. When at market, they can choose to buy less expensive food, basic goods, and raw resources for personal use. This results in a 50% savings of the list price in the PHB. This covers leather and padded armor types as well. A rural human can always find a material for a sling, a quarterstaff, or a club at no cost.
• Rural Work: Rural humans are generally aware of human activity when they encounter it. On a roll of 1 through 3 on a 1d6, a human will know if other humans were responsible for logging, mining or farming. Further, on a roll of 1 through 2 on a 1d6, they will know, in general, if the area is good for one of those activities. On a roll of 1 on a 1d6, they can even isolate specific resources such as high quality farming soil, mineral wealth for mining, or high quality lumber for specific purposes.
• Bonus Proficiency: If non-weapon proficiencies are used, they can choose to gain a bonus non-weapon proficiency from the following list: Agriculture, Animal Handling, or Mining.

Urban

Suave and cultured, cunning and conceited - even the lowest of the low has a hustle. Urban humans tend to be more culturally aware and savvy, and careful of word and deed when they want something. Politics comes as second nature.

• Ability Score Adjustment: +1 to Charisma, -1 to Strength. Urban humans typically represent guild members, members of the Church, and nobles. They tend to learn how to navigate social encounters early on, giving them advantages to their Charisma score. However, most of their time is spent around tables or markets, and not the back breaking labor of rural or coastal humans. This reduces their overall strength.
• Cityslickers: Urban humans tend to be wealthier than other humans and demihumans. When rolling starting gold, they add an additional die before multiplication.
• Social Intuition: Urban humans can give a person a quick one over and know where they stand. On a roll of 1 - 3 on a 1d6, an urban human can assess a person's social standing. On a 1 - 2 on a 1d6, they can determine the amount of wealth that the person has, and on a 1 on a 1d6 they can assess how that person will respond to their own goals and political maneuvering. Alignment is not the question - only the actual actions of the other person. As an example, a merchant is planning on using his friendship with the Princess to secure a wool shipment contract. Concerned his plans might be known, he looks around before slipping in the back entrance of the palace. A chaotic evil bandit is hiding behind a barrel focused on getting drunk in an alley, and the merchant immediately dismisses him. A neutral good human guard has been paid by another merchant to keep an eye on him, and he senses that the guard is being shifty and following him. Concerned he keeps walking, and plans an alternative approach.
• Bonus Proficiencies: If non-weapon proficiencies are used they can choose to gain two bonus non-weapon proficiency: Etiquette and then they can choose one based on their background. If they are a guild member, they can choose a crafting skill from the General table (i.e. blacksmithing). If they are noble or a church member, they can choose heraldry, reading/writing, or an additional language. Merchants can choose from either one. [/list]

Coastal

Coastal humans are like the sea, a smooth surface with depths underneath. As with the sea, they can go from calm to raging and back to calm fast enough to make your head spin. Coastal humans are those involved in making their living from the sea or other bodies of water, be it in an urban or rural environment. If not sailors, they'll own taverns or work the docks.

• Ability Score Adjustment: +1 to Dexterity, -1 to Charisma. Coastal humans typically represent sailors, fishermen, boat builders, dock workers, and those who make their living in or around the coast. Docks or decks tend to be crowded and subject to rapid changes, making coastal humans naturally quick on their feet. However, the blunt requirements of the job, and the totalitarian nature of ship organization, makes such humans blunt and crass, and quick to anger.
• Quick to Anger: Coastal humans are used to brawls, fights, arguments, and more. Like the sea, storms brew up quickly, cause intense damage, and disappear almost as quickly as they arrived. They never shy from a fight, and are quick to join in. Over time, coastal humans learn many dirty tricks in fighting. When fighting unarmored using traditional sailor weapons (club, chain, belaying pin, dagger/knife, gaff/hook, hand/throwing axe, harpoon, scourge/whip), they gain a +2 to their AC, and a +1 to their attack rolls.
• Weather Eye: Coastal humans grow up with a weather eye, and are generally aware of what the weather will be and when it will shift (i.e. the old saying "Red in the morning, sailors take warning; red at night, sailors delight."). Even if not aware they are doing it, coastal humans will be keeping an eye on the weather subconsciously. On a roll of 1 - 3 on a 1d6, they'll be aware that a weather shift is coming ("A storm is coming" or "The storm will clear") in the next six hours. On a roll of 1 - 2 on a 1d6, they will know the specific timing of changes within 30 minutes. On a roll of 1 on a 1d6, they will know specific conditions of an upcoming storm: lightning, hail, wind bursts, tornadoes etc. On a 1 on a 1d6, they can predict the outcome of weather magic based on the changes happening to the atmosphere around them. How they use that information is up to them.
• Bonus Proficiencies: If non-weapon proficiencies are used they can choose to gain access to a bonus non-weapon proficiency from the following list: Fishing, Rope Use, Seamanship, or Swimming. Coastal humans also gain weathersense.[/list]

Wild

Wild humans are from beyond the rural environment. Lost tribes in the mountains and jungles of the world represent wild humans. Typically, they have a lower technology level compared to civilized humans - though this isn't always the case. Wild humans will tend towards tribal organization in small bands.

• Ability Score Adjustment: +1 to Wisdom, -1 to Charisma. Wild humans typically represent tribesman, or cultures that eschew the type of urban environment found in compact cities. They might represent pioneers or colonists, or hermit sects that have isolated themselves away from the main population. They tend to be patient and observant, learning from nature and the wildlands, while also eschewing contact with outsiders.
• Weapons in Hand: Wild humans need weapons to survive, and even children are trained on specific weapons from a young age. Choose from one of the following weapons: bow (short), club, dagger, javalin, sling, or spear. The character is proficient in that weapon and can make a stone or bone version (Wild Weapon, Wisdom -3).
• Survivor: Wild humans grow up learning how to live off the bounty around them. They are skilled survivalists and foragers, and can generally find everything they need close to hand. On a roll of 1 through 3 on a 1d6, they can find enough food and water to keep one person alive for a day after one hour of foraging, hunting, or setting traps. On a roll of 1 through 2 on a 1d6, they have a general direction sense, and can figure out how to get somewhere with no trails. On a roll of 1 on a 1d6, they can determine the best way to navigate difficult to pass terrain. 
• Bonus Proficiencies: If non-weapon proficiencies are used they can choose to gain access to a bonus non-weapon proficiency from the following list: Animal Lore, Direction Sense, Fire-building, Running, or Mountaineering. Wild humans gain Survival automatically.