The whole “amount of genders” debate in America is so weird to me as Swedish speaker where for ages there have been four grammatical genders—now five, and newspapers and dictionaries use them formally.
Four utrum genders (men (“han”), women (“hon”), animals (“den”), and, for the last decade or more, non-binary people, or just unknown gender like when you’re listening to a story about an angry customer and the narrator hasn’t gotten to the the gender part yet and you wanna ask a question about the person (“hen”)), and one neutrum gender (objects (“det”)).
That is like so well established at this point that the whole “there are two genders change my mind” thing is almost confusing. There were four for hundreds and hundreds of years and sure, adding one more was controversial AF (I mighta been on the wrong side of history on that one ♫whistles innocently♫) but the controversy was so many years in the past now. It’s just everyday basics.
Of course those five categories are only rough estimations and for students of Swedish you are gonna have to sort every word. Like the sun is a “den” gender and a clock used to be a “hon” gender, I think it’s switched to “den” gender though, and a train is a “det” gender while a car is a “den” gender etc etc.
Memory guy Dominic O’Brien once suggested, for French learners (where there are, or were, just two genders), that as you create two cities in your imagination and as you learn new words you picture them in the correct city. I guess for Swedish learners you’d need five cities. “OK, so salt is ‘det’ gender, so I’ll imagine my friend in New Jersey putting salt on the table (table is another ‘det’ gender word)”.
That’s not to say that pronouns are an open class of words. Indo-European languages break down if closed classes are treated like open classes, if function words are treated lexically.
Things definitively can get added to them—for example, the preposition “at” (in the context of users at servers) were added to Swedish in the nineties, just as “hen” was a few years later—but those additions require somewhat of a critical mass. If you need non-standard endophora, I have sympathy but it’s gonna be difficult. However, the critical mass doesn’t need to be universal; your language just needs to work in your own community bubble. As in, as long as your friends and family understand you, you’re golden.