Running projections core outside of the ES

I very much like how the ES does projections. However in many cases I’d like to be able to create projections that are stored outside of the ES.

Is there anything (or is there anything planned) to allow projections persisting state externally? Say, referencing the cassandra js lib and using cassandra, etc?

I understand that I could build my own projections core by subscribing to streams, but the possibility to re-use an existing awesome functionality (including the ability to add/pause/reset projections) would be great.