I actually really like names like summer, autumn, river, rain. Idk why because I feel like they should be bad names.

I have had students or known kids named Princess, King, and Legend. Charleigh Brown. Atilla Hunt. Harley Quinn. Their name just becomes the collection of phonemes you use to address them. It’s funny when you see it, then you don’t even think about it anymore.

Mercedes was a name before being a car brand.

Mercedes is a Spanish female name, derived from María de las Mercedes ("Our Lady of Mercy" or "Mary of Mercies"), which is one of the Roman Catholic titles of the Virgin Mary.

https://en.m.wikipedia.org/wiki/Mercedes_(name)

Emil Jellinek was an Austrian-Jewish diplomat based in Nice who ran a profitable business selling cars, and, as a racing enthusiast, had been racing DMG automobiles under the pseudonym Mercédès, after his daughter, Mercédès Jellinek.[2] Later he contracted with DMG for a small series of dedicated sports cars containing an engine that officially bore his daughter's name.

https://en.m.wikipedia.org/wiki/Mercedes_(car_brand)

Yeah but naming someone after something from an unfinished series is still a very bold move. I could imagine a world where I name a kid after a fantasy character, but definitely not one where I name them after a fantasy character from an unfinished series. I mean, who knows where things might go!

Not inherently, no but you should never pick a name from any sort of media until their character arc is complete. Khaleesi was arguably fine until the character became an incestuous genocidal maniac.

That is now a major part of what people remember about the character. Just like you probably wouldn't choose to name your child Hitler now, the name is linked with a certain individual and their actions even though the name had nothing to do with it.

When you pick a specific name or word from fiction that doesn't otherwise exist, that link becomes even stronger, because that's the only source.