What to do when you've completed a Crusader Kings 3 campaign? You could start over as a new ruler in a new place, I suppose, but isn't that negligent? A truly responsible leader would steer their nation through the middle ages, the Victorian era, and World War 2. And you can do that thanks to a tool which transfers a CK3 world into Europa Universalis 4, picking it back up in the Paradox grand strategy game set in the following era. You could also then use another tool to take that from EU4 into Victoria 2, then into Hearts Of Iron 4 with yet another tool, converting the same world across one millennium-long 'megacampaign'.
Made by fans who did this with Crusader Kings 2 before, the CK3 To EU4 Converter lets you generate a new EU4 starting scenario from a CK3 save. It'll try to convert your map, countries, cultures, religions, rulers, heirs, advisors, vassals, empires, and a few other bits as best as it can to fit into, y'know, a different game with different rules. It cannot be exactly the same, but it'll broadly reflect the state of the world.
After that, you could use another of the fan team's tools to do the same going from EU4 to Victoria 2, and another from Victoria 2 to Hearts Of Iron 4. The gang are also working on one to go from Imperator: Rome into Crusader Kings 3, extending a megacampaign even further.
Before you ask, yes, someone else has made a HoI4 to Stellaris converter. That picks the top few nations from the end of WW2 and give them their own space colonies to start with, which even the creator cheerily conceded is "actually quite pointless". That hasn't been updated in a while so I'm not sure it works with modern versions of Stellaris, but I admire the idea.
You can download the CK3 To EU4 Converter from its Paradox Forum thread and GitHub, or if you subscribe on the Steam Workshop that'll offer an up-to-date list of the latest downloads (though not actually download it). The gang released the first version of the CK3 converter at the end of September, and are still plugging away at updates.