I don't mean a global culture, we need to keep seprate culture, that's what makes each part of the world awesome and unique, I meant every country forming under one banner but still mainting some self ownership, kindal Like america, all the states make there own rules but they all follow the policy of the white house, except rules in imporant matters are on a fedral level, and only minor things should be a state level matter, each state would have there own culture and traditions, but would follow the lead of the centralised global policy that's decided by the population of earth but argued by a group of people made up of representatives from each states, there would be no "prime minister or president" and only things could pass if 80-90 percent of the people agreed (on the planet) the job of the people in the group is to argue for and against policy to try and convince the global population which way is good and to also discuss and bring to the table problems.
I don't think we will ever have global tollerance, it's almost an evolved thing for us to hate some one or somthing just for being slightly differant, hence if a hostile alien species attacked we would instantly flock behind a single banner, those that don't would die leaving just the people who want to fight together simply becuase there attacking and there differant.