A common enemy? That is such a stone age mentality. If we absolutely need one here is a suggestion : human stupidity. And the most striking example of human stupidity is war. Because it is an event where one side will end up losing their life way before the time , while the other side becomes a murderer. Not exactly a win - win solution.

I think we can end all wars , forever , and never look back.

Instead we simply create a global super-society where every person without exception wins and there is no need any more for enemies anywhere. Instead we can simply live life to its fullest. Loving every second of it.