True, you never know for sure. But given the nature of American racial and social hierarchies it would be most likely that biracial Hispanic/Whites would identify as whites and that light skinned Hispanics generally will start identifying as whites. Many biracial Asian/Whites will have an upper middle class background and people with high social status are generally perceived as whiter than people with low status.
I agree. The category of "whiteness" has ever changing boundaries, many of which are ridiculously arbitrary. It could essentially swallow up several categories of people in the future who are currently excluded. Especially since "whiteness" is basically associated with the "mainstream" in the USA, and an increasing number of people without 100% European heritage are helping to define the mainstream nowadays.
And of course, we have no idea whether racial identity will have the same sort of impact on people's view of their place in the world in the same way 30 or 40 years from now. You could certainly make the case that it's gotten less important in the last 30 or 40 years, so who's to say it won't get less important still in the future?