Category Suggestion

How do I suggest a category. For example, "Health" is missing from a regional catgory relevant to one of my sites, and there is no other relevant category for me to list in?

You don't .

Just suggest your site to what would be the parent category. By all means add a note to the editor at the end of your description [like this]. If a Health subcategory is justified, s/he'll create one. That'll normally happen only if there also several other health related websites there.