If you're bringing these US Citizens into your country to get their skills, you want them working in jobs where they'll use their skills; or, you want them creating a startup where they can use those skills.
Requiring a job or getting an approved startup idea are both viable routes in the vast majority of countries in the EU, to my knowledge.
And, if memory serves, most people can get citizenship in those aforementioned countries in 5-6 years if they play correctly; and, many countries allow the US equivalent of a green card in a couple.
It's already pretty easy to move to Europe for knowledge workers.
Yeah I don't think actual citizenship is in the 'critical path'. You just need to make it really easy for people to move over and not be in a precarious legal situation.
