Jake Martinez said:
"What ratios of representation should we have in films?"
Something in the neighborhood of what the ratios are in the population at large.
In the context of the setting of the media.
Jake Martinez said:
"How should we decide or deduce what these ratios are?"
By looking at census data(for the population at large), and tracking similar data in films.
In the context of the setting of the media.
Jake Martinez said:
"Who should be the people to be in charge of deciding this?"
Society at large. This is a social issue, and so should be handled socially. Organizations focused on particular demographic groups should be tracking representation of their group, and mentioning it publicly when things seem out of whack.
Socially run companies are known as the government. Do you really want the government to run entertainment?
Jake Martinez said:
"How do you enforce some system like this?"
Social pressure. Mostly just asking representatives of those studios which have screwy representation "what's up with that?" every time you talk to them, and occasionally saying "good job" to those with good representation.
I prefer to buy media I like, not media I find meets someone else's social criteria.
Jake Martinez said:
"Does this change over time? How do we change this?"
Hiring practices. If things are massively out of whack, it's reasonable to assume there's some unconscious bias in the hiring process. Deliberately engage in preferential hiring to counter that unconscious bias until you're in the right neighborhood.
Which will soon be overturned by those who were not hired due to their demographic rather than their merit. You would be engaging in genuine discrimination in an effort to counter it. You would be creating victims to replace victims at a 1:1 ratio.
Jake Martinez said:
"What happens if you just really want to make a character that is of a race/gender that is already over represented?"
Then you ask yourself why you want that character to be that race/gender. If there is some significant story/characterization reason, then you go ahead regardless. Otherwise, you change it.
I'm not about to dictate the story someone else wishes to create because it doesn't fit my world view.