Is there going to be a sixth warrior cats series by Erin Hunter?

Yes! Its called Dawn of the clans! The first book is out and is called the sun trail.
Edit: The Sixth Arc is to be titled A Vision of Shadows. The Fifth Arc is Dawn of the Clans. The First book of A Vision of Shadows is to be titled The Apprentice's Quest and will be released on March 15th, 2016.