I just published an article where I try to analyse what important words are missing from various HSK levels. “Missing” here is defined as being very common, but yet not on HSK. To qualify for the list, a word needs to be twice as common as indicated on HSK, so if a word is on frequency rank 1-300, it’s considered missing in HSK if it’s not on HSK 1-3, which covers twice that much, or 600 words. I then did a bunch of manual sorting and culling, because comparing word lists is very tricky (most results you get are thing that are not actually words and are missing from HSK simply because the compliers of that list ignored them for that reason, such as 这个 or 哪些). Here’s the article:
I also put together a deck in Skritter you can use to easily study words that you really should know, but might have missed if you focus heavily on the HSK decks. See it as the unofficial companion deck to the standard HSK decks, if you will. Here’s a link to the deck:
There is also a version for TOCFL, although I used a different frequency list which leads to different results.
Skritter deck for the missing words in TOCFL