In pretty much every game I’ve played that had non-humans, humans have been boring in comparison. They’ve been the norm, the standard, the whatever, the “we’re kinda alright at everything” option. The mashed potatoes of species. That’s boring as shit, especially when you consider that in many settings they’re a young people but have done a lot in very little time. They’re described as curious, ambitious, industrious, and bold, but that’s not conveyed in their mechanics. You have a world where there’ve been civilizations for tens of thousands of years, create amazing stuff and then these primates show up and catch up in no time flat? I’m not sure how +1 to all ability scores and maybe a couple more skills reflects that.
Humans have got to be like the other options in that they have a direction, they have flavor, they have something special. I figure a big bonus to intelligence, bonuses to crafting, engineering, and lore skills, and some bonus languages. Give them some at-will spells if their intelligence is high. Humans are scientists, they’re engineers, they’re builders, they’re philosophers. Dwarves build because it’s the finest form that stone can take. Elves abstain from building because the finest form has already been achieved in its natural state. Humans build to do shit. Elves look at the stars because it’s pleasurable to. Dwarves look deep into the earth because it’s pleasurable to. Humans look everywhere to do shit. They want to peel the sensory layer off of the world and get at that sweet, sweet underbelly of reality. They want to know it, they want to understand it, and they want to use it.
They enjoy the crafts of the dwarves and the arts of the elves and various other things about all the other peoples of the world, but they will soon pass them by. It won’t take long, and it won’t be good for the non-humans. And it might not be good for the humans, depending on what’s waiting for them out there beyond what we can see and touch.