We have an open critical beta blocker issue that aims to make the entity storage controller able to automatically generate the entity schema for all defined fields.
An initial implementation plan was laid out in Prague (see section Unify Field storage). I am going to work on it the next week at DDD Szeged and I sketched a more detailed plan of attack at:
https://drupal.org/comment/8552811#comment-8552811
Feedback is badly needed so that I don't waste a full week of sprint time working on something that might not be good enough (or at all).
Feeback especially welcome from @chx since there are some aspects that might be controversial, if I recall correctly his position. Moreover I need help figuring out how the proposed solution would play with Mongo.