What will define the 20's in the US?

I agree Climate change and its immediate global effects worldwide will get pushed more and more to the mainstream where even conservative Republicans will eventually see the logic in considering adopting a more positive attitude towards renewable, "green technologies" solar, wind-powered alternative power sources, but if GOP starts hoping on board especially if they see the long-term profit potential, they'll try and corpratize it, make it another 21st century big business model.

Marijuana will be legalized federally and with legalization, means it's a Wild West-sort of capitalist enterprise towards companies competing against one another until the big pot growers and distributors become the same corporate raiders Bill Gates, Steve Jobs, Steve Wozniak, and Paul Allen swore they'd never emulate like their 1950s parents but did anyway with the invention of the dot.com Internet software revolution.

I think you might polygomorous relationships or maybe marriages(basically menage a troes relationships where 3 or more people are in a functional, committed platonic or sexual relationship) becoming not a movement yet, but I could see a trend developing where more people might experiment with it or there's more willingness in the mainstream media to discuss or talk about it.

In terms of international geopolitics: region of Caledonia in northeastern Spain is probably going to win its independence or gain greater internal sovereignty, for those who are unaware, Caledonia, historically, culturally, politically, even linguistically has always viewed itself as distinct and separate from rest of Spain since early modern period, this struggle has persisted for centuries from Moorish period to Spanish Civil War to Franco's dictatorship to Caledonians essentially last year voting in a non-binding referendum, to secede from Spain and form their own, independent sovereign nation.

France might turn more right-wing and nationalistic and develop a more skeptical, perhaps hostile attitude towards EU seeing how Boris Johnson Tories won a landslide election mandate over the question of continuing Brexit as policy. It won't happen overnight, and I don't see a lynchpin lightning rod for controversy like Marie Le Pens Front Nationale, she doesn't have the skills, savvy, and political charisma of an idealogue like Nigel Garage and her family's history especially with its confirmed links as Nazi sympathizers, her father was an accused and convicted Nazi collaborator during the Nazi-puppet French Vichy collaborationist regime(That part of French history is a dirty, dirty, vicious endless recriminating period for both French historians and citizens, it makes a good part of post-WWII French history seem tame, less contradictory and simple to figure out by comparison, I don't think there's ever going to be some sort of national collective sense of closure over the myriad of difficult, dark compromises and complex moral/ethical quagmires so many French feel their leaders, neighbors, friends, family did to one another just to survive). Modern French politics has always been characterized as chaotic, cutthroat, and ultra-competitve and usually hyper-partisan in a way that makes the current political divide in US still seem quaint.

Germany might be willing to become more then just Europe's strongest, most diverse economy. I could see them bulking up more militarily in response to a perceived sense of renewed fear or apprehension of Russian territorial or economic expansion. For some EU countries who have very long, vivid memories, a stronger, more militarized Germany with some of the world's most advanced, sophisticated minds and technology might make them a tad uneasy, especially if you're Poland, Hungary, or Austria.

Japan might adopt more of an assertive, military presence in Asia, especially in dealing with a rising superpower in China and perhaps India, although Japanese-Indian relations are far better and not adversarial. Japan will never be a nuclear power, the anti-nuclear sentiment is way too strong there but Japan has always had a strong warrior tradition with its own medieval customs, code of conduct, ethos, varying philosophies, that mentality hasn't disappeared, it's just been re-purposed or re-directed towards economic superpower since end of WWII. End of WWII and resounding military defeat didnt end or discredit Japanese nationalism, it's still a potent, powerful force amomg Japanese right-wing political parties.