LocalLLaMA

Uh I forgot Llama3 has 8B parameters. What about something like L3-8B-Lunaris? Though, that's not the latest and greatest anymore and it's tuned for roleplay. Maybe it's worth a try, but there are probably better ones out there. I use Mistral-Nemo-Instruct-2407 for pretty much everything. I think it's a great allrounder and can do anything from answering questions about facts to dialogue to storywriting, and it's not censored at all. But it has 14B parameters unless I'm mistaken... Does your worldbuilding have to be fast? Because if you're fine with it being very slow, you can just run it on the CPU, without any graphics card. I usually do that. It'll take a few minutes to ingest the prompt and come up with an output. But I don't really care for use cases like storywriting or creative worldbuilding. (Software would be something like llama.cpp, ollama, LocalAI, koboldcpp, ...)

Otherwise I think you'd need to find a fine-tune of a <=8B parameter model that fits. There are enough of them out there. But I found writing prose, or story arcs is a bit more challenging than other tasks, and I believe worldbuilding might be, too. So I guess it's not as easy as finding a random roleplay or chatbot model.