But seeing how it's implemented, I think they could totally integrate something like FSRS to at least just replace their scheduling (how long until an item is next shown). The unlocking system can be implemented as a separate gatekeeping mechanism, and the buckets can be coded for certain step thresholds (instead of wanikani's "stage").
Basically, this is their entire srs system: https://docs.api.wanikani.com/20170710/#spaced-repetition-sy...
https://github.com/inro-digital/simple-tools/tree/main/packa...
YMMV though, since I haven't user-tested it in my app yet haha.
TBH though, I think for a true WaniKani competitor, need to reserve a decent % of work for building the dataset. Putting together all those mnemonics, cleaning up definitions, defining the order of introduction of characters and choosing words... is a lot of effort. There's a reason that WaniKani's so generous with pretty much their entire platform, but specifically states in their docs that the mnemonics/hints data doesn't belong to you... it's a large part of the effort.
Although, I guess could be integrated into the 90% work if you make sharing mnemonics into a community effort.