[Chaos]

The only races that SAN is really 'useless' for are the Corrupted. Every other race can gleam something out of SAN, whether they have the racials for it or not.

I also have to question why APT isn't used in Quick to Master (i.e. +(Scaled APT/10) SP), but I digress.

The only thing I can think of that would give SAN use for Humans while keeping them within their lore would be an optional Trait:

Divine Boon
REQUIREMENTS: 100 God Loyalty, 20+ SAN

Humans, lacking a patron god of their own, are especially susceptible to divine gifts. As long as you retain 100 Loyalty to your god, you will gain a racial skill, based off your chosen god. (The skill's effects are based off your SAN. If God Loyalty falls below 100, the racial skill is removed.)

I do not have any concrete ideas on what each God would grant at this time.