"You might as well try to custom build your own AI for your story purposes"
Really? I'm too dumb to do that.
For starters, you're allowed to use chatgpt to formulatw the conversations
You write what you characters would ask to the system, and you put them into chat-gpt to see what they'll say.
Most of the time, the answer from chatgpt is way way better and more advanced than average system/ai from stories, because gpt is a real life supercomputer ai afterall.
This is my most reccomended way to everyone. Your "Fantasy system" should at least match the qualities of chatGPT interactiveness, gone are the days of old boring stat only system that can't even chat. Even chatgpt is better than the old chatbot.
You can also try to find online roleplay AI. They'll play a badly or generic characters, but good for inspiration of an gpt style charaterized AI would look like.
And the best part is, if you're willing to learn from sketchy tutorial and put some hobbyist effort, you can download AI like chat gpt into your desktop computer, laptop, or your phone.
Because you'll install the AI on your device, you don't need to worry about internet connection anymore. Not unless you want to use your AI to browse the Internet, which is actually not that hard to do, spent the effort and you can do it too.
Sure the quality may drop and slower than chatGPT, the AI may hallucinate more often than GPT, and forgot about the data you inputted more often than GPT, but you get AI installed on your own device!
Which also means, no censorship, or you can remove them if there's any, depend on which AI you download.
There's totally no reason for fantasy world, especially scifi or urban ones, to not have interactable system, when practically every human with internet from 2022 can have professor smart AI anywhere from their device.