There is a very large set of possible types that may be used to pass data between elements. Indeed, each new element that is defined may use a new data format (though unless at least one other element recognises that format, it will be most likely be useless since nothing will be able to link with it).
In order for types to be useful, and for systems like autopluggers to work, it is necessary that all elements agree on the type definitions, and which properties are required for each type. The GStreamer framework itself simply provides the ability to define types and parameters, but does not fix the meaning of types and parameters, and does not enforce standards on the creation of new types. This is a matter for a policy to decide, not technical systems to enforce.
For now, the policy is simple:
Do not create a new type if you could use one which already exists.
If creating a new type, discuss it first with the other GStreamer developers, on at least one of: IRC, mailing lists.
Try to ensure that the name for a new format is as unlikely to conflict with anything else created already, and is not a more generalised name than it should be. For example: "audio/compressed" would be too generalised a name to represent audio data compressed with an mp3 codec. Instead "audio/mp3" might be an appropriate name, or "audio/compressed" could exist and have a property indicating the type of compression used.
Ensure that, when you do create a new type, you specify it clearly, and get it added to the list of known types so that other developers can use the type correctly when writing their elements.
If you need a new format that has not yet been defined in our List of Defined Types, you will want to have some general guidelines on media type naming, properties and such. A media type would ideally be equivalent to the Mime-type defined by IANA; else, it should be in the form type/x-name, where type is the sort of data this media type handles (audio, video, ...) and name should be something specific for this specific type. Audio and video media types should try to support the general audio/video properties (see the list), and can use their own properties, too. To get an idea of what properties we think are useful, see (again) the list.
Take your time to find the right set of properties for your type. There is no reason to hurry. Also, experimenting with this is generally a good idea. Experience learns that theoretically thought-out types are good, but they still need practical use to assure that they serve their needs. Make sure that your property names do not clash with similar properties used in other types. If they match, make sure they mean the same thing; properties with different types but the same names are not allowed.