The Mesos JIRA Road Map provides a view of which tickets are targeted for upcoming releases.
The ‘Epic’ issues in JIRA provide a view into the larger projects planned to be worked on.
The Road Map Wiki captures the big ticket items for the project.
There is also an ad-hoc Release Planning document that attempts to capture times and features for upcoming releases.
If you have comments or suggestions, feel free to reach out to the Mesos dev list (email@example.com).