There are a few categories of potential changes to Horizon. Bugs are problems that need to be fixed. Blueprints are feature specifications. These are features with a bit of thought to them that are ready to be taken to implementation. For good ideas that aren't ready to be championed to implementation by you or someone you're associated with we have this wish list.
A wish list item is a feature you'd like to see happen to Horizon but isn't ready to happen yet or you aren't ready to make happen. It should include a short description. If more details is available it can be linked off to (possibly written up as an etherpad document).
When something on the wish list is ready to be implemented it will be moved to the appropriate place, for example it could become a blueprint.
- Example: Add a cornify button to Horizon (http://www.cornify.com/)