I have to say this is a great question. And I really like Bart's point. We only divide by race or maybe by national origin nowadays, but originally, you were english, or french, or norse (viking) [all of which would be germanic], or a hun, or mongol, slav, slovak. greek, dorian, czech, arabic, egyptian, caananite, turk, kazakh, uzbek, ukraine, lienz cossack, etc. this is a longer list, but it's the same idea. these were all tribes of people with different cultural and physiological makeups. some settled an area to the point that the people became a nation, and some went even further, with the nation becoming a political state, but they all began as a people, a tribe, a separate kind of human, more or less like a subclass of homo sapien sapien, or cro-magnon man which is what we are.
Of course I'm also intrigued about the idea of humans who have no great physiological differences standing out as special for something other than mediocrity. I would probably go with something like they are smarter, or are better with some aspect of invention. maybe humans are the best toolmakers. or philosophers/thinkers. or they make the best peddlers and merchants. the best drivers and sailors. maybe they are the most skilled fighters (not the strongest, but the best with tactics and strategy, and/or the best with physical use of the weapons). the list could go on.
Regardless, the idea is that just because humans don't get bonuses on their abilities, such as str, dex, con, and so on if we are talking in terms of dnd, should we let that prevent them from having any outstanding features? If they have balanced, mediocre stats, give them a skill that no one else has. Or by the same token change the core dynamics of the game. if humans can't be the strongest, smartest, sturdiest, dextrous, wisest, or any of the things that are based on physical traits and developments because somebody has those traits, and they can't have a predisposition towards magic, the only non-natural, nonphysical gameplay dynamic, because the elves have that, create a separate force of nature which is nonphysical, which can compete with magic. preferably something which does not overlap or mimic magic's purpose in the scheme of gameplay dynamics. one example could be that humans have an innate ability to manipulate time, or maybe they can tamper with the fabric of reality, changing what is real and distorting what can and can't be done/exist/occur you get my drift.
While I would personally still go with the concept of sub-subspecies of humans, I would probably add some aspect of the second and third choices to give a little more oomph to the race.
This all comes together to make the standard being stand out. Now humans can still be the mediocre race they are and benefit from not being something else. The fact that adding a new game dynamic wouldn't affect the fact that humans would still be "standard" and would still suffer from ability mediocrity really adds to my liking of that third option, at least philosophically, because it works well with the other two to make a balanced change to the paradigm of humans in fantasy rpgs, and it also satisfies my wish to keep humans standard and mediocre, without dooming them to reduced potential as a powerful, useful character.
