Right now, we're starting a working group of interested people to talk about getting all of these APIs right. You can see an initial thread at https://forum.torproject.net/t/defining-an-interface-to-arti/6432 with links to different design sketches; pretty soon we hope that there will be a new subforum to work on the issue and discuss more.  I'll follow up with a link once there is (unless somebody else does).