I have created a new media/format of presenting video, but is not a 'technology' but a concept, a design system. I want to release the idea for free for anyone to develop, use etc. What kind of license should I use?
When it comes to copyright, your specification and the code that implements it are two completely unrelated things. This means that whatever copyright license you choose for your specification, it has no effect on the license an implementer can use for their code.
If you want to enforce an open-source implementation of your specification, then you must look to contract law (and probably a very strict closed-source license on your specification to prevent further distribution).
On the other hand, if you want others to freely build upon your specification and you are not really bothered if someone creates a closed-source implementation, then you could choose an open-source license for your specification. For example one from the Creative Commons family.