I'm part of a group that has written a tabletop RPG with this purpose in mind. In addition to its use as a playable game, the world guide is meant to offer ideas and inspiration to writers. There's a lot general descriptions of the forces of change that occur over a hundred years.

We're not quite done, but the nearly finished draft is already available for free on our website: https://fullyautomatedrpg.com. Take a look and see if you find anything that's useful to you.

The main geopolitical change is that oil is not going to be anymore a king-maker. I would expect a second Arab Spring (though the Arab world is more than just the Arabian peninsula) with more success. I would expect a lower influence of Russia as well.

There are two possible flavors of future and really, both are credible, it depends on the stories you want to tell: more integration in supra-national entities or more independent states.

1. More supranational structures: EU of course and the African Union are the obvious things, and, dare I say, there are interesting spy stories/James Bond plot to be done in the context of building these powerful entities. The emergence of a new democratic federation of Russia after a period of civil strife can be an interesting plot. The MERCOSUR and the ALBA can show interesting interactions. When it comes to Asia, I suspect one may need to invent a new organization, integration of countries are not on the program (except for Korean reunification). Also, do not forget the UN. It could consolidate in a form of government. It already has a de facto minister of trade (WTO), of labor (ILO), of health (WHO), of education (UNESCO). It is a slow process but solving the climate crisis could have given it the political credit it is currently lacking.

2. More regionalism: With a lower incentive to move where labor is, people instead chose their place of living according to acquaintances, culture, religion, political affiliation... It led to an increase demand of separationism. And with less time wasted in labor, more people got involved in politics and interested in their local laws and rights. Also, several "virtual lands" started appearing with people not claiming a specific territory but participating in a government of their choice and living as a foreigner in their country of residence (If that seems weird, search for the Sovereign Military Order of Malta, it exists today). Many big blocks exploded: China and US are now mostly a loose federation of hundreds of independent entities. Old countries still exist but are only considered like the king of Belgium: folklore from another time. What matters is that you are a member of the Episcopalian Ecologist Alliance of Baltimore or from the Geek Order of the Lemmy Budapest Fortress.

I tend to find 1. more utopian but 2. far more fertile for ideas! Note that the 2 can be combined: federations become bigger and bigger but also looser with local independence becoming more real. To give you an example, the Basque country (a bit like Catalonia) is currently split between France and Spain but with these two countries becoming part of the EU, the border splitting the country is now irrelevant. Paradoxically, the existence of a supra-national entity may give independence movements access to a lot of their demands, but stop short of sovereignty.

The Lost Cause by Cory Doctorow has a similar albeit IMHO a bit too US centric setting. Worth a read never the less.

I think it would be interesting to write a story from the perspective of a young second generation climate refugee in northern Europe.

I've been doing a worldbuilding exercise for awhile, where I photobash together an image of a solarpunk scene and write some commentary about it. Maybe it'll help? https://jacobcoffinwrites.wordpress.com/postcards-from-a-solarpunk-future/

I tend to write with the expectation that things will get worse before they get better. I have a couple reasons for this: 1, I think there are likely some hard times ahead and I want the folks there to have optimistic fiction that makes concrete suggestions for improving things. 2, it's a useful way to make changes in the setting. Why have rural areas contracted into denser villages with more wild land around them? The car/oil infrastructure became unreliable and a lot of houses in the endless sprawl became impractical or too expensive to commute from. Why are they prioritizing public transit? Because the roads were bad before the crumbles, they got worse in the bad days, and the populations have already mostly resettled in denser clumps that are a good fit for trains, ropeways, etc. Stuff like that.

I tend to be a bit more concerned with day to day life, but I've been doing a fair bit of research, so if you find yourself looking for ideas on what a solarpunk apartment building, city street, rural homestead, or other location might look like, feel free to reach out, I've got tons of ideas.