Allow to create Epics between Themes and User Stories

As a product owner working with product backlogs, I usually work with three levels of specification: Themes, Epics and User Stories. This a very common way to structure a product backlog.

So, it would be great to be able to start a backlog only with Themes and Epics, and as the project evolves through backlog grooming of prioritized Epics, User Stories would start to be written.