There is no perfection in politics. One cannot create a perfect nation. The American constitution is quite good. It's main fault is that it didn't explicity provide states the right to secede.

To answer your other questions... I would support a strict immigration policy. Only those who have a track record of attending conservative religious service should be allowed to immigrate. I would also want some kind of a test for intelligence for immigrants. What happens outside of my nation is none of my business, so I have no problem with ethnostates (like Israel).