So, there's another project – called kvrocks https://github.com/apache/kvrocks , which is for people which don't need in-memory perf of redis but would like to use its protocol and be still very performant. However, its devs never packaged a release in deb and other formats.
So, if you were to implement something similar ^ which will implement the protocol, have ability to grow on disk, and be available in distro repos (ideally) – would definitely hold a niche of many little projects which benefit from redis-like thing I've done.