A world in which countries have no borders, country's names are just ways of identifying land areas, and humanity has merged into one giant mixture of peoples and cultures. Everyone is free to come and go as they please.
Could this ever happen? Ever?
If so, when and how? In stages, or all at once? Could technology help to address concerns over security, resource allocation, etc.?
I'm hoping it happens in the 22nd century, but I suspect it will take much longer to come about, owing to cultural differences and ingrained prejudices. Current economic blocs will probably do it first, and it's already possible to a certain extent with the EU. But the kind of world I'm talking about would be even more porous, i.e. doing away with all passports, visas, or other travel documentation, except perhaps in exceptional circumstances.
