An app that plays the audio of Japanese words from WK in Firefox and requires the English meaning to be typed would be great. There’s already an app for that in Chrome (/t/Is-WaniKani-not-hard-enough-for-you-listening-comprehension/1148).
Also, something like Kaniwani but without the SRS would be great - an app that just prompts for J-input in response to an English word.