you cannot copyright ideas

I don’t know about US law, but in Europe you certainly can, and it’s an issue over and over again ending up in courts.

simply mentioning Zaphod Beeblebrox doesn’t trigger anything to do with copyright

Yes it does. Fanfiction e.g. is considered infringement of the creator’s right, and that doesn’t extend to the exact verbatim text but to general plots, names, etc. It’s even infringement if you write a story about „Härrie Pötter“, since it’s immediately obvious that it’s based on Harry Potter.

Some years ago a German discounter sold a costume that was an obvious reference to the TV depiction „Pipi Langstrumpf“, a famous character by Swedish author Astrid Lindgren. Mind you, in essence it was just a really cheap wig and a dress somewhat resembling a tv character. The company owning the rights on the character sued and won.

Edit: oh! I missed the part where there were two courts that decided it was infringement but in the end the highest court overruled that and said it’s not: https://www.lto.de/recht/hintergruende/h/bgh-urteil-pippi-langstrumpf-romanfigur-urheberrecht/

Which only proves that all of it is completely arbitrary and just a matter of opinion. /Edit

In another case, someone took a photo of a soldier, cut out the soldier, turned it into an outline, and printed and sold t-shirts of that. If you took the shirt and put the photo next to it, it was immediately obvious it was based on the photo. Here the court had no issues, because in their opinion it was too far away from the original work to be compared with it. 🤷‍♀️

So, it’s quite impossible to draw a line between an idea and an implementation, and that’s why thousands and thousands of infringement cases are ending in front of courts, and in the end the only relevant factor is the opinion of the court.