You can always use plugin types like 'local' or 'admin tool', they are general purpose plugin types. They do not provide you with any starting point like mlbackend plugin types does though, they are just a way to plug any external code into moodle's core.
Ideally analytics API and machine learning backends would support unsupervised learning and reinforcement learning but they do not and it does not seem like HQ is going to put resources on this at short term. In any case we are open to contributions from the community and any proposed API specs will be welcomed. They would be used as a starting point for future implementations.