I guess the answer depends a lot on how big your scope is. For any given topic, you will find people disagreeing if you look hard or long enough. And simultaneously, almost any topic can be a source of unity for a group of people, if your scope is small enough.

I thought protecting our livelihoods, on which we depend with our very lifes, should be a clear and uncontroversial unifier. But apparently climate change and mass extinction are controversial topics for some people. While at the same time, these topics create some very tight knit communities for other people.

But maybe you are looking for answers like: Music, dancing, sports, humor, shared values, shared experiences.