I’m personally a big fan of iKnow. It replicates the SRS function of Wanikani/oragnizes your study for you, but is more robust than Wanikani (and most other vocab apps?) in what it quizzes you on.
Its sole disadvantage, IMO, is that I don’t think it’s truly accessible for people starting from scratch. If you’re lower-intermediate and looking to pick up more vocabulary as a routine, though, I’d really recommend it.
It is another subscription (or lifetime purchase) on top of WK, but if you personally enjoy the way WK is structured, I think it’s a fairly worthwhile combination.
Edit – Oh, apparently they may have nixed the lifetime option. I’d still look into it though.