Atawans Guide on How To Make A nimi sin™
there are 4 qualities that i believe all nimi sin should meet, especially if they want to fit in within the rest of toki pona. they are:
1. Be Universal
any random person on earth should be able to know what you are talking about. a toki pona word should be so simple as to be understood by all people. barring that, you should at least be able to explain what the concept is with minimal confusion. this rule eliminates most highly specific nimi sin, as well as any units or culture-specific ideas.
2. Be Physical
this is the rule where most serious nimi sins fail: every toki pona word is tied to a physical process in some way. this is one of the most important ideas of toki pona, because it ties in with the sort of implied culture that toki pona creates. there are a few exceptions, like the word weka, which arent quite physical, but for the most part this rule holds.
3. Be Useful
this is basically the test to see if your nimi sin should be allowed. if its not useful enough, people wont use it. obviously, most pu words break this rule, but a few others break this as well.
4. Be Independent
this rule is a bit more subjective: basically, if your word can already be made out of other words, its probably not worth adding. you can sortof mix/match this with the rule above: a particularly useful word doesnt have to be that independent from other words. this makes a sort of "convenience factor," which can be a little subjective, because people have different opinions on how much convenience should play a role in toki pona. for me, as a benchmark, i usually say that if you can express the idea in 3 words, its not really different enough to be worth it. of course, many toki pona words break this rule anyways, which is why its the lowest one.
these rules are sorted in priority order, so the top one is the most important.