It’s not that hard to imagine either. My mind immediately jumps to a sort of post-apocalypse where climate change caused the governments of the world to collapse. Sounds bleak at first, but you can make the setting solarpunk by showing how people adapted to live in the new world sustainably, and are thriving in the absence of oppressive hierarchies maintained by corporations and states. The external threat could take all kinds of forms, but I think the most compelling would be a cyberpunk city-state that survived the collapse and is now attempting to re-establish governance over the thriving solarpunks.