Caution: Thread drift ahead.
A nice idea, but you probably want to change "people livingin in the US" to "persons with income higher than x".
No. That would still be a progressive tax system. My idea is the one, true fair way of taxing, at least according to my pipe dream.
Essentially, everyone living (as a legal resident) in the US has the same opportunity to benefit from our system of government. Since we all have the same benefits, we should all pay the same for that benefit. That includes children, the unemployed, rich, poor, even Mr Tactical Scapegoat. This tax system would instantly put everyone on "budget watch" so that the FedGov can't get away with as much free-spending of our money. Also, there would be no way for anyone to deduct, capital-loss, business-expense, or otherwise write-off their way out of paying taxes.