So, I’m not a terribly big fan of level limits. I think the usual arguments for them… that otherwise demihumans would dominate, and that they provide balance… are specious, at best. So, like a lot of people involved in 2e, I get rid of them.
But that raises the question of “Why humans”? The dubious benefit of can advance to any level gone, humans have nothing. So, I prefer to give them something. My choices are
+1 to Charisma
+1 to all saving throws
+1 WP and +1 NWP, without class restrictions.
Charisma is harder to powergame, and fits with humans being able to make large and diverse societies in the ways others cannot; not pairing it with a penalty is an added bonus.
+1 to all saving throws makes humans a little more resilient, in the same way dwarves and halflings resist magic and poison, humans are a little bit more resilient than one might expect.
+1 WP and NWP. These are analogous to elven and halfling combat bonuses, or special skills like stealth. Humans approach things differently, and have some unexpected skills, but don’t necessarily have the SAME unexpected skills.
It gives humans a mechanical reason to be played, when the mechanical “advantage” of “can be any level” is removed.