Features and improvements are released as soon as they are completed (more or less) as part of the weekly releases. The seasonal releases are basically summaries of changes in recent weekly releases, i.e. their content is not determined in advance. 

For forward-looking information we maintain product roadmaps. These are often presented at conferences, seminars etc, and work has been initiated to make them more visible here on EPiServer World.

But the intention of the continuous release process and semantic versioning is that it should be simple to update to the latest version at any point. Moving to a new major version may mean more work is required, depending on what features and APIs the implementation is using. Moving to other versions within the same major version should however be simple. So I would suggest you plan to move from 9.x to 10.x at a time that suits you, and once you are on 10.x updating to get any additional features in 10.x you desire will be a smaller effort.