Hi David thanks for posting on the community!

We do not currently have a Six Hat template but we do have several brainstorming templates for Lucidchart along with a wide variety of additional templates that can be found in our Diagram templates gallery. Please submit your suggestion using our feature request form so that your feedback can be based onto our product development team for their consideration. In the meantime you can create a Custom Template following the instructions found on our Work with Templates article.

I would note that if you are interested in collaborating and brainstorming with others I would highly recommend our whiteboarding software -- Lucidspark. It is optimized for the ideation and creation state of a project and has a wide variety of collaboration and facilitator tools for working with others.

Hope this helps!