“I wonder what an RPG would look like where there were literally no assumptions left unaddressed.”

Eventually this breaks down. Language is built on assuptions, e.g. I have to assume you know what I mean by “assumption” in order to say this sentence. (There’s a related concept where the fact that game instructions are supposed to be talking about a “working system” helps lend clarity — like when someone is teaching you something like a math technique and you can tell that you’ve understood it because you can see how it works, or “cryptic” furniture assembly instructions that snap into place mentally as the parts snap into place physically). I think a big stumbling block is people figuring out when it’s necessary to start a new mental model and when to piggyback on an old one. Boardgames tend to send a lot of signals that say “this is a new game that you haven’t played before”, while RPGs aren’t always like that (especially for people who don’t want to read rules, etc.).