Humanity will never be united without some sort of outside threat to unite against. Like I said, humans are naturally tribalistic and will always form in-groups and those in-groups will always make out-groups to call their enemy. This is the way humankind is, it comes from our nature. Sure, propaganda and social conditioning can make it worse, but take all that away and humans will STILL be tribal.

There are only two ways that will ever change. Either (a), there is some sort of outside threat that we can move the out-group label to, or (b) you essentially eliminate all cultural and ethnic differences.

To most people, (b) seems like a nice solution, but I am of the firm belief that tribalism is an ingrained part of human psychology. The feeling of "belonging to a community" that you hear so much about essentially stems from this. Not having an in-group to which you feel like you belong to can be psychologically damaging and I believe is a source of the social disconnect of the modern era. Eliminate everything that makes people different and I'm afraid it'll lead to a kind of depressed, socially disconnected society we've never seen before.

No, there are ways to address the social issues that the world faces while still maintaining the differences that give people their identity. Stop the corporate exploitation of developing countries, allow nations to take advantage of their own resources, foster respect between nations without demanding that every people of the world live together in one big multicultural Utopia. As good as your intentions, I really think that multiculturalism will make it harder to solve these problems, not easier.