A simple language, skills and work experience requirement is the key approach.  Trying to be too restrictive with education, phd, funds, etc, requirements would be foolish and counter-productive.  The hungriest Americans who would be most beneficial to a European economy are the 25-45 age group with 4+ years of experience in their field.  

If possible, favor single people by not offering visa's to the spouse/children or whatever.  Those people can be granted other means but if your goal is emigrant brain-drain you need to get the young, mobile and energized Americans who are uncertain about their futures here.  There are many and they can offer you as much as you offered the US these past 200 years.