Update: To clarify, Turing_Machine likened the suggestion that California be given to Mexico to alephnerd's description of the complex and unclear proper ownership of the Koh-I-Noor. I claim that the situation in California is simpler, at least in some places, because there are native Californians to give stuff back to and multiple tribes are willing to cooperatively take custody and stewardship of the land, as the InterTribal Sinkyone Wilderness Council has. This is in many ways the opposite of Pakistan, India and Afghanistan having competing / mutually incompatible claims.
edit: the OP is actually a good example now that he explained it.
What is now California was never controlled by what is now Mexico until the Spanish colonizers showed up. The Aztecs (who were themselves brutal colonizers, as it happens) weren't running the show there or anything like that.