Bunpro.jp is a pretty great alternative for something like this if you’re not already using it for the (really excellent) grammar practice. They have pretty extensive vocabulary decks with lots of the gaps not here on WK- including more tailored ones (both official and fan curated) that feature all sorts of individual topics like onomatopoeia, dialect variations, daily situations (like grocery shopping or driving), terms used in specific media, etc.
It functions on a similar SRS system as WK and has tons of example sentences usually too. There’s even WK integration so you don’t have to relearn material already practiced here. If you’re not already using the site it’s very feature rich and gets high recommendations from me.
I do agree, though, that some sort of content deck system here on WK would be an awesome addition since I do prefer the flow, interface, and structure a little more. I had an idea like this in the past myself; Love letter to Wanikani - #4 by drackyslime