Oops, I want back to edit and forgot to write the rest of that paragraph.
I was going to say, supporting borrowing means you need to retain all the borrowable word forms - nouns, adjectives, and verbs - which rules out some extra-radical possibilities, like making a language where verbs are a closed set and actions are represented by nouns. But to my knowledge no natural languages do that, so that's not much of a restriction.
I think that most of the potential lies in the "extra-radical possibilities". The traditional linguistics descriptions (adjectives, nouns, prepositions, and so on) don't seem to apply very well to any of my word languages. After all, they're just a bunch of natural language components; they needn't show up in an artificial language.
For example, in one of my word languages, there's no distinction between nouns and adjectives (meaning that there aren't any nouns and adjectives, I guess). To express the equivalent of the phrase "stupid man"...
Rationality requires intelligence, and the kind of intelligence that we use (for communication, progress, FAI, etc.) runs on language.
It seems that the place we should start is optimizing language for intelligence and rationality. One of SIAI's proposals includes using Lojban to interface between humans and an FAI. And of course, I should hope the programming language used to build a FAI would be "rational". But it would seem to me that the human-generated priors, correct epistemic rationality, decision theory, metaethics, etc. all depend on using a language that sufficiently rigorously maps to our territory.
Are "naturally evolved" languages such as English sufficient, with EY-style taboos and neologisms? Or are they sick to the core?
Please forgive and point me towards previous discussion or sequences about this topic.