Greetings, Russell.
I would think the ESV API would be a good one to model for developing a new API, for lots of existing tools likely already know how to build queries to interface with it. There could be an additional query parameter for specifying the version (OWB, WEB, KJV, etc) but the rest of the parameters could remain the same, that is the ones that are relevant.
I was talking with Crossway to look at the development of an ESV API 3.0 that would be an implementation of our Open Scriptures REST API specification, but this hasn't yet materialized. I'm very keen to see this work move forward, but with recent (happy) birth of my second child, I'm now very much daddy mode.
Cheers,