So, equivalent to the population of Liverpool, each and every single year. Who is going to pay to build a new city every year? Are the poor migrants going to pay to build these cities? Even if funded, could we even build that quickly? It's not "straight forward".
Why stop at 500,000? Since you are plucking figures out of the air, why not 1,000,000 a year? Why not 2,000,000 a year? Why not 5,000,000 a year? 700,000,000 people around the world live in extreme poverty. Should we have open borders and invite them all?