We have recently discovered the power of traits and are starting using it.
Currently, we make a region in a bundle for traits and simply create normal entities.
We manually replace "class" by "trait" and add the "use" in the required entities by hand.
It all works fine, except, "trait" is always replaced by "class" on export to ORM.
Our basic need would be to be able to tag an entity as trait, so the the class becomes / stays "trait", this could be a simple attribute on the entity which puts trait instead of class during export.
More extensive support would be to actually use the traits in other entities, but this could come later.
Doctrine2, Symfony3, PHP 5.6+