The question is, what is a 20th level human?
I guess the question that must be asked in order to answer that question is "what makes humans unique"? What does "humanness" mean relative to everything else?
To a certain extent, it is pretty much impossible to try to keep humans as a totally generic race in a system with 20 race levels. They need to have
some concept or else they would join the 3E Fighter as a 20 level progression of feats, skills, and not much else, and I don't think that is exactly ideal. Humans would need to provide a viable alternative to other options, and mechanisms like "one bonus trained skil" or "a bonus feat" can only go so far.
There are a few major options I can think of:
1) Humans as a genuine jack-of-all-trades. Rather than just providing bonus feats to passively support this concept, it could be pursued more aggressively, with a 20th level human having a wide variety of abilities designed to let the character perform almost any kind of action. Not just bonus skills, but bonuses to
every skill. Ability to equip every form of weapon and use any form of equipment or magic item. Experience with all forms of crafting and knowledge. Basic spellcasting, combat ability, and sneakiness. Some non-trivial chance of succeeding at any task, whether that is "convince the king to help us" or "cast
Meteor Swarm". I'm not sure of the best mechanical implementation for all of this, but it is certainly one possibility.
2) Level 20 humans are demigods. Basically, raising your human level makes you more like Heracles. It doesn't provide much in the way of skills, techniques, spells, or powers, but it does go a long way towards letting you have a 34 in every ability score.
3) Linking this with the idea that elves and dwarves are subraces of humans that I mentioned above, and going with the idea that racial level goes up in addition to class level rather than instead of class level... Humans would be the masters of, well, the humanoid form. Generally, that means things we tend to take for granted, like having opposable thumbs, having language, being able to read, being able to sing, wearing clothes, having two arms, and so on. The unique advantages of humans as opposed to other races depends a bit on what other races are like, but it would certainly include basic stuff like "can wear armor" or "can carry weapons". Raising the level of human thus improves the character's general skill at these sorts of tasks. Inhuman races would, in turn, improve things like their skill at tearing a foe to pieces with claws and fangs by raising their racial level.
With regards to option #3, the best implementation of "race matters" I've seen is in certain entries in the SaGa series of videogames, where racial choice matters a lot and there really is no class choice. In those games, other racial choices include robots (whose stats are entirely dependant on equipment and can't use magic), monsters (gain strength by eating other monsters and don't use equipment), and mystics (mutant psychics with innate spellcasting ability). In that context, humans are distinct as the characters who carry weapons, cast spells through acquired knowledge, and improve weaponskill through acquired knowledge. More equipment-dependent than monsters or mystics, but make use of skill more than robots. It's a very different kind of racial set-up than most, and shows off the natural strengths and weaknesses of humans rather well.
